Why do you think people get so uncomfortable and defensive when people talk about God?

Why do you think people who do not believe in the Christian God get so uncomfortable, defensive and worked up when God is brought up? You can casually bring up something about and its no big deal, but bring up God and its another story. Why does it bother nonbelievers so much if they think God is about as real as Santa Claus? I don't get defensive and bothered when a little kid brings up Santa.