Why do we say "bless you" when someone sneezes?
The Real Answer
It dates back to at least the 6th century when Pope Gregory I ordered people to say 'God bless you' during a plague, possibly because sneezing was an early symptom of illness. Other theories suggest people believed the soul briefly left the body during a sneeze, or that the heart momentarily stopped. None of these are medically accurate, but the social habit stuck and evolved into a purely automatic response.
We've been running a 1,400-year-old program that nobody knows how to uninstall. Someone's nose malfunctions for half a second and every person in the room owes them a verbal blessing. Skip it once and suddenly you're the one with the character flaw.
Why People Ask This
This is one of those things you've done your entire life without ever questioning it — until suddenly you do, usually mid-sneeze when someone doesn't say it and you feel the social awkwardness. Most people assume it's religious, which is partially right, but the real surprise is how many completely different cultures independently invented their own sneeze responses, which means we all collectively agreed sneezes are a tiny emergency.