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".
Coughing or sneezing people say God Bless You because they want you to be healthy and have God do it by blessing you. Peace
When I was very young, I asked my cousin why people always said God Bless You when you sneeze and she said your heart stops so by saying that God may or may not allow your heart to beat again.
Ti means that good gift of God