Though there are general categories, of course, there will also be those unique angles that make the situation on one end of the spectrum (learn from it and "solve" it or is a reflection of the cheater and cannot be fixed, not a one time thing).
The most standard that I have witnessed is that the guy doesn't feel there is any spark anymore. Regardless of his self-esteem he will want to feel wanted, desired. Once we get into that sharing a place phase of a relationship we all too often settle in and just nest. Both sides need to make some effort to keep the spark. No matter how attractive you are your partner "needs" a little variety now and again.
I dated a woman that had everything figured out. She never gave a man a second chance. Her reasoning being that "my man gets all he needs at home." Oh my, having dated her I knew first hand there was no hyperbole to her comment.