Why does my friend think every girl is hot?

He wasn't like this when we first met and I do have a boyfriend. However, he ogles women and says nasty comments about them and used to say things like that to me before I told him to stop. So I basically wear huge sweaters around him so he can't see anything. He's just a pervert and I have teased him about it. For the most part, he's a smart guy and can be nice when he wants to be. But, my question is why does he have to ogle every single girl? And don't say it's because he's a man! There has to be more to it than that....Also, I like him as a friend and I'm just curious why he does this.