Obviously there is no one easy explanation here (as you can see by all the different responses).

I think evolutionary factors play into a general affection people have for boobs. They are humans first source of food and with it an extremely intimate contact. Also on a “birth readiness” perspective Boobs on a woman suggest that she can feed a child (just as wide hips suggest she can bear one).

There is something to be said about culture. Since boobs are considered “sexual” they have to be covered in “decent” settings (see also discussions about female presenting nipples on social media) which simply adds to their sexualization.

Also, in my sexual experience (Bi male) nipples and breasts (boobs/man boobs …) are erogenous zones so playing with them (touching/pinching/kissing/biting…) actually is arousing thus they are “sexual” parts.

Thats just my two cents.