Why do some guys like butts so much?

Because they do?

Don’t sweat this. As long as they’re not obsessive, rude, creepy, or rapey with others about their preferences, file this under the “people are delightfully weird” category in your brain.

And lest you think this is an exclusively straight male predilection, there are plenty of people (women and other genders included) that like a nice man’s, woman’s, and person’s derriere.

Butts are “a thing” to a great many kinds of people.