The common denominators amongst most cheaters I know are a certain lack of respect for themselves and/or a certain lack of respect for everyone else. And I say that because at the end of the day, when you're in a monogamous relationship and you get into bed with a third person behind your love's back, you've made a decision about your partner's life: that they're going to be someone who has been cheated on. Cheating is always a choice. Perhaps not a well-thought out one, but a choice nonetheless.
Also, it never seems to have symmetry in feeling: The enjoyment the cheater gets never seems to match the despair the cheated feels, in depth nor duration. I just don't see how that could be worth it to someone. Just break up or grow a pair and have an adult conversation about the terms of your commitment. The fact that life-long monogamy is generally unrealistic in human terms is no justification for trouncing someone's trust. When one wants to move on or open up the relationship, communication is the only thing that can ease us out of those broken hopes.
I think a lot of people cheat because they want to move on but have an ironic fear of hurting feelings.