Most Helpful Girl
Well I don't think there's one general answer to this because it will be different for each individual's situation. However, I personally feel the fact someone cheats says a lot about the person they are and how seriously they take relationships. It also shows how mature they are in my opinion. They could of communicated any problems and worked on them, or broke it off if they weren't honestly in it 100%, but they choose to betray someone they "care about".
With that said I'm more inclined to think in general they want a quick hook up. If not that, they may realize the mistake they made and wish they had what they once had. Maybe there are more reasons and maybe there are people who don't fit into my generalization, but this is my opinion based on my own experiences.