Why do guys like butts?
Well that's a profound question and it has more to do with emotion and hard wiring than it does with logic. There is a wired in preference in men for an hour glass shape in women. Size doesn't really matter that much if the proportions are waist being 70% of the size of the hips. But in general it all goes back to the proportion of the hips and the musculature of the glutes all signaling a healthier woman for child baring. Not unlike women having a tendency for strong arms and a V shaped back in men. Primates in general have a visual acuity for butt since quite often it heightens in coloration when a female of a species was receptive to mating. It's also been theorized that the reason breasts and cleavage in general is so similar visually to butt is to capitalize on that attraction to butt, so men would visually have something to look at during intercourse in a frontal position.
Now that I threw in so much evolutionary science in answering the question, it completely dulls the reason for it all. This apparently is one of the most viewed thread in the forum... That should substantiate the science.
It IS a profound question

Re. primates acuity for butts: not just coloration, but also swellings in some species (see chimpanzee)! Problem here is that humans have a perfectly concealed ovulation, i.e., females don't advertise unlike many other species that they are receptive, at least not with visual cues (behaviorally they very well do).
Re. breasts and cleavage: way too teleological an argument to fit in an explanation based on evolutionary biology. Don't forget the main function of mammaries that dictates evolutionary aspects of their development!