Either you like them and get jealous or your pride can't take being cheated on. Add on insecurities, then you get an obsession.
But honestly, it's never happened to me. I always accept that 1) they may cheat and 2) if they cheat, it's over. Just like that. It ain't worth my time obsessing over stuff. Even if I were to date a guy who was a notorious player, I'd accept our relationship as a one-time chance that would quickly be over if he stepped out of line.
I don't understand women who stay after a man cheats and keep trying to
make him be faithful, however that works.

If he wants to cheat, he will. You can't stop him. What you can do is leave. And if you aren't strong enough to do that, then he obviously has full control of the relationship and you're truly his bytch. Not his woman, his bytch.