Most of us were raised with them. They are eye catching, and it almost attracts us in an instinct sort of way. Not too mention they are incredibly soft. There's a bit more reasons, but those pop up in my head.
Usually theyre on display by the ladies choice of outfit so it'd be rude not to look and make anyone we know aware. Its common courtesy. I don't really get glazed over though their just boobs like I've never seen those b4. Plus I'm more of a butt guy. Though the principles are the same.