It use to be thought that when you sneeze you were a little close to death so people would say "God Bless You" as a way of wishing you well. When you sneeze, particularly if you have a sneezing fit, you don't get enough oxygen and your heart sometimes skips a beat. That is why people thought it was dangerous to sneeze so they would wish for you that God would intervene and save you. So they would say, "God Bless You".
