system
A chat on subreddit /r/me_irl
human
Cultural taboos. You don’t generally see fetishization of breasts in cultures where they aren’t usually covered up. 

A century ago, women’s *ankles, calves, and necks* were the focus of men’s obsession, because they were usually covered up and “off-limits.”
gpt
They're also secondary sexual characteristic, so men are genetically hard-wired to find them enticing.