Why do guys obsess over girls butts?

I've always wondered this. I mean it's not like the butt is part of the vagina. It's just there to provide extra body fat for child birth. Some guys I know seriosly are in love with girls butts. It's kinda weird because I don't do that with guys. (and also, some of them like nasty out of shape and jiggly butts that some girls flaunt like it's a good thing. Yuck. If they are obviously out of shape some of my guy friends find their huge jiggly butts attractive. What is wrong with a small firm butt??)