Why do people cheat?

If you fall in love with someone else while you're in a relationship with someone why don't people just break up with the person and go out with the new person? To me, cheating is a lot worse than ending it with the first person and getting with the new person. Sure either path will lead to someone getting hurt, and it's not so easy to end a relationship after being with someone for a while, but cheating seems like the more painful road. If the love just isn't there anymore, save yourself and your partner the trouble and just move on.