Why do people bless us

when we sneeze?

from Wikipedia: Sneeze:

Onomatopoeia
Some common English onomatopoeias for the sneeze sound are achoo, atchoo, achew, and atishoo, with the first syllable corresponding to the sudden intake of air, and the second to the sound of the sneeze.

A similar linguistic approach has been taken with several other languages; in French, the sound "Atchoum!" is used; in Finnish "Atsiuh!"; in Icelandic "Atsjú!"; in Swedish "Atjo"; in Danish "Atju!"; in Dutch "Hatsjoe!" or "Hatsjie!"; in Hebrew and Lithuanian "Apchi!"; in German "Hatschie!"; in Estonian "Atsihh!" and "Aptsihh!"; in Hungarian "Hapci!"; in Polish, "Apsik!"; in Russian , "Apchkhi!"; in Turkish, "Hapşu!"; in Italian, "Etciù!"; in Spanish "¡Achís!"; in Portuguese, "Atchim!"; in Romanian "Hapciu!" ; in Malayalam "Thummal"; in Filipino "Hatsing!"; in Japanese, "Hakushon!"; in Tamil, "Thummal"; in Kannada, "Akshi"; in Korean, "Achee!" and in Vietnamese, "Hát-xì!!". In Cypriot Greek, the word is "Apshoo!", incidentally also the name of a village, which is the cause of much mirth locally.

In Howards End, by E.M. Forster, a sneeze in polite society is "A-tissue" - a literary allusion to its respective remedy.

Historic instances and practices
In the Hellenistic cultures of Classical Antiquity, sneezes were believed to be prophetic signs from the gods. In 410 BC, for instance, the Athenian general Xenophon gave a dramatic oration exhorting his fellow soldiers to follow him to liberty or to death against the Persians. He spoke for an hour motivating his army and assuring them of a safe return to Athens until a soldier underscored his conclusion with a sneeze. Thinking that this sneeze was a favorable sign from the gods, the soldiers bowed before Xenophon and followed his command. Another divine moment of sneezing for the Greeks occurs in the story of Odysseus. When Odysseus returns home disguised as a beggar and talks with his waiting wife Penelope, she says to Odysseus, not knowing to whom she speaks, that "[her husband] will return safely to challenge her suitors"". At that moment, their son sneezes loudly and Penelope laughs with joy, reassured that it is a sign from the gods (Od. 17: 539-551).

In Europe, principally around the early Middle Ages, it was believed that one's life was in fact tied to one's breath - a belief reflected in the word "expire" (originally meaning "to exhale") gaining the additional meaning of "to come to an end" or "to die". This connection, coupled with the significant amount of breath expelled from the body during a sneeze, had likely[citation needed] led people to believe that sneezing could easily be fatal. This theory, if proven conclusively, could in turn explain the reasoning behind the traditional "God bless you" response to a sneeze, the origins of which are currently unclear. (see "Traditional Responses To A Sneeze" below for alternative explanations). Sir Raymond Henry Payne Crawfurd, for instance, the late registrar of the Royal College of Physicians, in his 1909 book "The Last Days of Charles II", states that, when the controversial monarch was on his deathbed, his medical attendants administered a concoction of cowslips and extract of ammonia to promote sneezing.[4] However, it is not known if this promotion of sneezing was done to hasten his death (as coup de grace), or as an ultimate attempt at treatment.

In certain parts of Eastern Asia, particularly in Japanese culture and Vietnamese culture, a sneeze without an obvious cause was generally perceived as a sign that someone was talking about the sneezer at that very moment - a belief that is still depicted in present-day manga and anime. In China, Vietnam and Japan, for instance, there is a superstition that if talking behind someone's back causes the person being talked about to sneeze; as such, the sneezer can tell if something good is being said (one sneeze), something bad is being said (two sneezes in a row), or if this is a sign that they are about to catch a cold (multiple sneezes).

Parallel beliefs are known to exist around the world, particularly in contemporary Greek, Celtic, English, French, and Indian cultures. Similarly, in Nepal, sneezers are believed to be remembered by someone at that particular moment.

In Indian culture, especially in northern parts of India, it has been a common superstition that a sneeze taking place before the start of any work was a sign of impending bad interruption. It was thus customary to pause in order to drink water or break any work rhythm before resuming the job at hand in order to prevent any misfortune from occurring.

The practice among certain Islamic cultures, in turn, has largely been based on various Prophetic traditions and the teachings of Muhammad. An example of this is Al-Bukhaari's narrations from Abu Hurayrah that the Islamic prophet once said:
When one of you sneezes, let him say, "Al-hamdu-Lillah" (Praise be to Allah), and let his brother or companion say to him, "Yarhamuk Allah" (May Allah have mercy on you). If he says, "Yarhamuk-Allah", then let [the sneezer] say, "Yahdeekum Allah wa yuslihu baalakum" (May Allah guide you and rectify your condition).

Traditional responses to a sneeze
In English-speaking countries, the common verbal response to another person's sneeze is "(God) Bless You" or the lesser common "Gesundheit" (the German word for "healthiness"). There are several proposed origins for the use "Bless you" in the context of sneezing:
Various alleged but conflicting superstitions relating the sneeze to evil spirits. This includes beliefs that a sneeze could release one's soul, thus leading to its possible capture by lurking evil spirits (as explained in the "Historic instances and practices" section above), or that the evil spirits could enter the body though the open mouth of a sneezing individual, or that the individual is sneezing out sins or evil spirits which had taken residence within the body and is thus in need of the blessing to prevent the exorcised spirits from re-entering the body. Some proponents of this last theory have further suggested that it was bad luck to open the mouth again to thank the person who uttered "Bless you" for fear of circumventing the original purpose of the blessing.[citation needed]
Some say it came into use during the plague pandemics of the fourteenth century. Blessing the individual after showing such a symptom was thought to prevent possible impending death due to the lethal disease.
Attempting to bring the sneezing individual back from the brink of death in the brief moment during the sneeze when the heart skips a beat, which otherwise leaves the sneezer momentarily in limbo between states of life and death
A polite way of congratulating the sneezer for his or her impending good luck as signaled by the sneeze.
In various other cultures, words referencing good health or a long life are used instead of "Bless you":
In Albanian, one says Shëndet (shuhn-det), to which typical responses are Faleminderit, meaning "Thank you", or Shëndet paç, meaning literally "May you have health".
In American Sign Language, it would be appropriate to do the Excuse-Me sign.
In Arabic (Levantine Arabic), the response is صحة (Sahha), which likely evolved from the word صحة (Sihha), meaning "health", or نشوة (Nashweh) which means "ecstasy". The response is either thank you شكراً (Shukran) or تسلم (Tislam/Taslam) which means "may you be kept safe".
In Arabic, the typical response to a sneeze is يرحمكم الله (yarhamkom Allah) and the answer is يهديكم الله و يصلح بالكم (Yahdeekom Allah wa yousleh balakom) or شكراً (thank you). However, it is more common to say Alhamdo lel lah i.e "All praise is for Allah (God)", to which people will respond Yarhamokomo Allah, meaning "God Bless you".
In Armenian, one says առողջություն (aroghjootyoon).
In Azeri, sneezing is usually followed by the response Sağlam ol, which means "be healthy". Sometimes, Afiat is used, similar to the Persian response.
In Bosnian, one says Nazdravlje, meaning "To your good health". The person who sneezed usually responds with Hvala, meaning "Thank you".
In Bulgarian, one says Наздраве (Nazdrave), which means "[to your] health" or "cheers". The person who has sneezed can then say Благодаря (Blagodarya), which means "Thank you".
In Chinese, one says (bù hǎo yì si) (Standard Mandarin) or (Standard Cantonese), meaning "excuse me" or "sorry".
In Croatian, Na zdravlje (almost always pronounced nazdravlje) ([to your] "Health") is said after a sneeze. For sneezer it is polite to reply Hvala meaning "thank you."
In Czech, one says Pozdrav Pánbůh, meaning in colloquial Czech "Bless God" or "Greet God".
In Danish, one says Prosit, from the Latin meaning "to your benefit". Originally used when toasting, today it is used only when someone sneezes.
In Dutch, one usually says Gezondheid (literally translated as "health") or Proost (which means "cheers", see Latin below).
In Estonian, one says Terviseks, which means "[to your] health".
In Ethiopia, one says Yimarih/Yimarish, which means "[May God]have mercy on you".
In Finnish, one says Terveydeksi, which means "[to your] health".
In French polite speech, after the first sneeze, one says à vos souhaits which means "to your desires". If the same person sneezes again, the second response is à vos amours, which means "to your loves." Santé ("Health") is the informal response.
In German, Gesundheit ([to your] "Health") is often said after a sneeze. This is also common in parts of the United States.
In Greek, γίτσες (jitses) ([to your] "Health") is said after a sneeze.
In Hebrew, one says לבריאות (livri'ut), meaning "to health".
In Hindi, one says Satyam (sat'yam), meaning "truth".
In Hungarian, one says Egészségedre!, which means "[to your] health".
In Icelandic, one says Guð hjálpi þér! ("God help you!"). There is also an old custom to respond three times to three sneezes like so: Guð hjálpi þér ("God help you"), styrki þig ("strengthen you"), og styðji ("and support").[5]
In Irish, one says Dia linn!, which means "God [be] with us!"
In Italian, one says Salute, which means "[to your] health".
In Japanese, a sneezer might apologize for the outburst, by saying ｷﾝﾜｹ (Sumimasen) or ｸｵﾜｵｽ (Shitsurei shimashita), meaning "Excuse Me". In formal occasions and less often within the family, after one sneezes, someone else blesses them by saying ｨｾ｢ｶﾉ (O-daiji ni), meaning "Take Care" in informal contexts and something along the lines of "Get Well Soon" in a more formal situation.
In Korean, a sneezer might say ?(Nuga nae yaegi hatseo), meaning "Did someone talk about me?" after a sneeze.
In Kyrgyz, one says Акчуч! [aqˈʧuʧ] (which may be based on an onomatopœia of the sound of a sneeze, like English "atchoo" discussed above), to which one may respond Ракмат!, meaning "thank you", if the person who said "акчуч" is liked.
In Lithuanian, one says Į sveikatą, which means "to your health". And person which sneezes answer Ačiū that translates as "Thank you".
In Macedonian, one says На здравје(na zdravje), meaning "[To Your] Health". The person who sneezes usually says Здравје да имаш (zdravje da imash) which means "Have Health [yourself]", or just says Благодарам(blagodaram) "Thank You" or Фала(fala) "Thanks".
In Maltese, one says Evviva, which comes from the Latin for "[He/She Is] Alive!".
In Norway, Sweden and Denmark, one sometimes says Prosit - Latin for "[May It] Advantage [You]".[6]
In Persian, if the sneeze is especially dramatic, Afiat Bahsheh (عافیت باشه) is said, "[May] Cleanliness/Purity be bestowed [upon you]".
In Polish, Na zdrowie ([to your] "Health") is said after a sneeze as is Sto lat ([I wish you] a hundred years [of health]).
In European Portuguese one says Santinho, which means "Little Saint".
In Brazilian Portuguese, one says Saúde, which means "[to your] health".
In Romanian, one says Sănătate ("[To your] Health") or Noroc ("[To your] Luck").
In Russian, the appropriate response is будь здоров(а) (BООD' zdah-ROV - to a male sneezer, or BООD' zdah-RO-va - to a female sneezer), which means "be healthy". For the sneezer it is polite to reply спасибо (spah-SEE-bah) meaning "thank you".
In Serbian, Na zdravlje (almost always pronounced nazdravlje) ([to your] "Health") is said after a sneeze. For the sneezer, it is polite to reply Hvala meaning "Thank you."
In Slovak, Na zdravie ([to your] "Health") is said after a sneeze. For the sneezer, it is polite to reply Ďakujem meaning "Thank You."
In Somali, one says Jir, which means "Live Long".
In Spanish, one says Salud, which means "[to your] health" and Jesús (in Spain) referring to Christ (similarly to God bless you in English).
In Tamil, one says Nooru aayisu for the first time, which means "(Have a life of) 100 years", for the second time it would be Theerga-aayisu which means "(Have) a Long life" and for the third time it would be Poorna-aayisu which means "(Have) a healthy long life".
In Telugu, particularly around the Indian state of Andhra Pradesh, the phrase is Chiranjeeva, which translates to "(May you be blessed with a) Life without death".
In Turkish, a sneezer is always told to Çok Yaşa, i.e. "Live Long", which in turn receives a response of either Sen De Gör ("[and I hope that] you see it") or Hep Beraber ("all together"). This is to indicate the sneezer's wish that the person wishing them a long life also has a long life so they can "live long" "all together". For more polite circles, one might say Güzel Yaşayın, i.e. "[May You] Live Beautifully", which may be countered with a Siz de Görün ("[And may You] witness it").
In Urdu, the response is traditionally Al-hum-do-lillah, i.e. "All praise is for Allah (God)".
In Vietnamese, the response is traditionally Sống lâu, i.e. "Live long" which, like "Bless You", is an abbreviation of "Wish you a long life" or "Lạy Mụ" /layh-moo/, from the culture of Middle Vietnam, meaning "Mother Nature blesses you."