Why Do Men Look Down At A Woman's Booty When She Walks By?

Some guys are attracted to butts and it's natural to want to look at things that are visually appealing. Most guys look at girls boobs. Though most also look at both. I have to admit, though, I also find myself checking out other girls boobs or butts. Maybe it's a comparison thing. I don't know. I'm not doing it for any sort of pleasure. A lot of girls do this, I'm beginning to realize.
 
What's your say on this?