I have no f***ingt clue why I like breasts so much, but I think to a degree straight men like breasts because they don't have them. If that makes any sense.
There's also some slight bits of evidence to suggest that bigger, rounder breasts are (or were at some point) an indicator of higher estrogen levels, which on a purely biological level would be a pretty good reason for a man to find someone attractive.
Typically, women who started puberty earlier will end up with bigger breasts, regardless of when their breasts stop growing. I'm probably a pretty good example of that.
Sidenote, a woman with big breasts typically has a big ass, which I also like, and hips, and all sorts of other deliciousness.