you should be able to get to the bottom of why he cheated. Maybe he was angry with you about something else and didn't know how to express himself, so he acted out instead. Maybe your relationship was on the brink of getting more serious and he got scared — and the cheating was actually a sign that he saw himself with you forever. Not only do you need to figure out why the cheating happened but what led to it in the first place and how you both may have contributed. Once you get to the heart of the matter — and neither of you may ever fully grasp all the reasons — you can start to work toward doing what it takes to avoid it in the future. Then, If you can't, you need to get out of this relationship and move on. And if you can, then you need to let him/her earn the trust back and start putting this relationship together again.