Why do people think it proper to say "God bless you" (or something like that) when someone sneezes?

it's an old superstition where your soul is trying to escape your body, and saying bless you chases it back in.

it's become an empty habit now, supposedly polite. which it isn't. not all people like hearing "bless you" constantly. not all religions believe in that.

From www.wikipedia.com

There are many theories regarding the origin of this custom.

One traditional explanation for the custom is that it began literally as a blessing. Pope Gregory I the Great (AD 540-604) ascended to the Papacy just in time for the start of the bubonic plague in AD 590 (his successor succumbed to it). To combat the plague, Gregory ordered litanies, processions and unceasing prayer for God's intercession. When someone sneezed (seen as the initial onset of the plague), they were immediately blessed ("God bless you!") in the hope that they would not actually develop the disease. Sneezing while alone necessitated saying "Bless me."

Not relevant to any papal directive, the phrase bless you or god bless you in Greek literature predates the ascention of Gregory.

"Bless you, my dear!" he said, and "bless you, bless you!" at the second and third sneeze.

-- Apuleius, The Golden ***, AD 150

Dick cannot blow his nose whene'er he pleases, His nose so long is, and his arm so short; Nor ever cries, God bless me! when he sneezes - He cannot hear so distant a report.

-- Greek Anthology, c. AD 500

Superstition

Other theories are based on superstitions and urban legends about sneezing and what a sneeze entails. Some well known superstitions that may have contributed to bringing bless you into common use are:

The heart stops when you sneeze (it doesn't), and the phrase bless you is meant to ensure the return of life or to encourage your heart to continue beating.

A sneeze is the expulsion of some sort of evil, and the phrase is meant to ward off the evil's re-entry.

Your soul can be thrown from your body when you sneeze, and saying bless you prevents your soul from being stolen by Satan or some evil spirit. Thus, bless you or God bless you is used as a sort of shield against evil.

A sneeze is good luck and saying "bless you" is no more than recognition of the sneezer's luckiness.

Alternatively, it may be possible that the phrase began rather uninterestingly as a response for an event that wasn't well understood at the time.

Another urban legend states that you cannot open your eyes while you sneeze, or if you manage to your eyes will pop out. This is, as stated, nothing more than an urban legend.

In the dark ages 1100 to 1400 ad..plagues ravaged european and third world countries. A sneeze was indentified as the start of the onset of some plagues. God Bless You was sometimes said as a part of the last rites. Also the german's had a saying for a sneeze i'm not sure how to spell it but genshudheitd meant "good health"

I have heard two reasons for this.

One: Your body actually stops working while you are sneezing so you get blessed in case you body just decides not to work again

Second: It is beleived that a spirit passes through your body when you sneeze so people say Bless you to send the spirit on the way.

The wish on sneezing is not only in English but also in French, German and I know of some oriental cultures. I don't know whether its correct or not but I was told in France that it was believed that with sneezing all the body-functions cease to work for a moment, and one is sort of reborn after that moment. As it was believed that if that moment of inaction stood extended, there would be certain death so people wished the person on his survival.

I think it is because long ago it was believed that the person or maybe their soul was, at some instant during the sneeze, vulnerable to evil and that was a way of helping or praying for them so nothing bad would happen.

I was told..

In Old Ireland, they immigrated here, they believed the heart stopped when you sneezed. So they said "God Bless You" to make sure it started again.

Just like Good Bye used to be "God be with ye"!

well i heard that when u sneeze, your heart stops for like half a second or something but im not sure if thats tru but thats y ppl say god bless u becuz its like ur heart stops.. some ppl say it becuz thats the way they were raised, to say god bless u... some ppl say it to be a kiss a_ss lol.. i say it just if i feel ike being nice or if it just slips out.. i dont say it to be proper.. so yea

The saying was invented because your heart actually stops beating at the time your sneezing

"God bless you" is an ye olde' tradition. for it used to be through that when you yawn or sneezed a devil could posses your body. So, if you had a blessing, it couldn't heppen.