Another reason is because the person isn't really attracted to their partner and seek someone else due to the Comparison Level which explains that if you aren't getting enough out of this relationship then you'll go to someone else. For example, most men (including myself) like a girl with a nice back that can be grabbed, if you're going out with someone that doesn't have these qualities, then you'll seek someone else that does have one. Of course, that's a little deterministic but in most cases, it is true.
Oh yeah of course the person who is in a committed relationship but chooses to serial cheat doesn’t have the problem! What rubbish. People can be very manipulative, especially if they’re in a relationship whereby the cheating partner has control - also may be a reason the brazenly cheat. I was just saying people taking back cheaters adds to the problem. But you saying the cheater doesn’t have a problem is just foolish. Sounds like something a cheater would say.