Most Helpful Guy
Could be a ton of things, but honestly it's not much different for the reason women stay with their "significant" other. The way I see it is getting what their significant other doesn't give them. Usually, when the "cupcake" phase ends they still like that person, but they want that feeling again. Sad to say, but it's similar to not getting rid of your favorite pair of shoes. This isn't the only answer though; everyone is different. For the record, I don't cheat, so my thoughts are purely on people I know.
Most Helpful Girl
Well just because you're cheating on someone it doesn't mean that you don't care about them. It could be that you're just looking for an extra piece of attention that you don't have. People learn to forgive others for the mistakes and move on with it, it means that the other must really be in love with him enough to do it