Why do guys like or are attracted by girls' butts?

This would almost be like asking why a woman might like a man's abs, arms, or his chest. You do realize women are often staring at men's butts more than men are at women's.
I'm pretty sure you couldn't answer most of those.

I will tell you why I like a nice butt on a woman, others may have different reasons. I like a nice butt on a woman because it emphasizes her figure, a nice butt will indicate she takes care of her body (Or blessed with genetics) It is the core of her sensual sexuality. It tells men whether she is fertile or not (A primal instinct, really)

Some of the same things can be said about a woman's breasts(Although not as appealing as the butt), like if they're medium to large sized, but perky breasts, also means she's taking good care of her body-adding to her sexual appeal.

Which brings me to my next point, it's not ultimately the butt I find attractive it's her figure as a whole, it has to have that curvy but proportionate look.

And men are not attracted to every woman's butt, I'm only attracted to a few, I can tell if a woman doesn't work out, and get a rough idea of their diet.

Well for me it's the whole thing about what I like about women in general. I like curves. I like the shape, I like how it moves, I like the way it sicks up on my Girlfriend when she's laying down, or how it sometimes jiggles in just the right way when she walks. If you want to understand why, I'm afraid I can't give you too much. I think it's more primal instinct. I see it and I think about my urges and what I'd like to do with the girl. But really I don't think it's just the ass. I think it's the entire lower body. Legs, hips, and ass. Especially if she's bent over. ...and back to primal urges. lol

Actually, it makes more sense to be attracted to butts. XD

If you've got a nice wide set of hips it means you'll be able to give birth easily. Boobs, on the other hand, work perfectly well no matter what size they are. =P

It's part of the whole golden-ratio, breeding-hips thing. The larger the back end, the more likely they are to be able to pass a large, healthy child. It's primal.

cause they jiggle :Dand if you're lucky, you got a nice handful for your mannn