I wish I could, but you ask the impossible. There are as many theories about that as there are slang names for boobs, and there are lots of those, trust me, because I know them all.
Some say it’s an attraction left over from when we were breastfed as infants. Others say it’s that breasts represent fertility. Some even purport that cleavage resembles a woman’s backside and reminds our inner cave man of the good old days when we could mozy up and mount you like a dog and get busy. I encourage you to research it and see what’s being said by people far more qualified than I am to answer the question. Me, I just like them because they’re soft and jiggly and fun to play with. And they look like big ice cream sundaes with cherries on top. I love ice cream.
As for the boob-starers, almost all guys will take a gander at your funbags, especially if you are rocking some cleavage (which I encourage, by the way). We are powerless over that shit. But I’m talking about quick, subtle (hopefully) peeks. Any guy that stares blatantly at them has decided, at least on a subconscious level, that he doesn’t care if you notice or if it makes you uncomfortable, which means he has no respect for you. In other words, he’s an asshat, and not worth your time. Walk away.
If Staring Guy is a co-worker and it’s happening on the job, ask him to stop, or at least embarrass him by looking down at your breasts to see what he’s looking at. If it persists, talk to Human Resources; they’re all ears when it comes to sexual harassment, and constant staring is sexual harassment.