Adept wrote:

To answer your question, this might sound perverted but whatever, here we go.
I think one of the reasons is when thinking of sex. Like with boobs you would be thinking of how you would like to do the foreplay and get the girl going or whatever. The ass is hot because you think of what it would look like during sex, like looking at it as you're fucking.

But I also think it's just in the male mind to like them, maybe to look for big boobs deep down means that the woman can feed their children better and the guy doesn't know it, or wider hips makeing it easier for child birth and giving the child a better chance of survival. But oddly enough I don't like them too big, I like the medium size, but I think it's just encoded into the male brain to like certain things to help the children survive better and be as healthy as possible, but not like that's what they conciously think about, they just like them.
Hope I could shed some light on this.