Infidelity?

I don't think there is a very reason for someone to be a cheat in the end or end up cheating each other. It all depends on how you handle the relationship. There might be common reasons like maybe love does not exist anymore to both parties but are afraid to admit it, it could be that they want to feel again the excitement they've felt way back then and etc. Whatever the reason is, it is the responsibility of both parties to keep the relationship and not end up as a failure.

To Many Reason why some relationship end up cheating each other. All I can do is to guess what are the most common reason. First is that One of them fall out of love, second is that either of them feels the emptiness in a relationship. The other are history.