Sneezing is actually the result of a person’s soul trying to escape from its fleshy prison.  Without a soul, a person will die, or, worse, be possessed by evil spirits constantly looking for any opening to take over a human and destroy the world.  The ony possible way of preventing these evil demon spirits from destroying the world is to force the soul back into the person’s body by saying “Bless you.”  By greeting the soul like a Hare Krishna at an airport, you can scare the soul back into hiding, saving the person from demonic possession.