Bless you!

Why do people say "bless you" when you sneeze? I mean, I know the whole thing about your spirit leaving in the sneeze and so people say "bless you" so the bad spirits can't enter. But really, why do we say it? It seems reasonable to excuse yourself when you burp or fart or something like that. Those are bodily functions that can be stifled or contained, and it's fairly rude when you allow these to happen in company. But sneezing is involuntary. In my case, I almost always sneeze twice in a row, and it seems overboard to "bless me" every single time I sneeze. Is it really polite, or is it just a force of habit for which there's no good explanation?