Why do people say?bless you"when you sneeze
Because people at one time believed that sneezing and illness was brought upon by evil spirits. So they say god bless you to help repel those spirits. In Germany they say gesundheit, it means: to your health.
It's a polite thing that people pick up.
The superstition was that a sneeze was actually your soul trying to escape. This would mean that you almost died which is apparently worthy of a blessing.
I once heard that it's cuz your soul is trying to escape your body, so you say bless you when a person sneezes. I was in kindergarten when I heard this from other kids. ;)
When we do a good thing for people
Actually it is because while sneezing your heart beat stops for 1 millisecond and this is why after sneezing it is said that God Bless You
It was once thought that the heart stopped beating when a person sneezed. Being blessed either helped the heart get going again or was a good way to start in the afterlife.
I tried to get people to say "congratulations" when we fart, but so far no luck.