Loneliness. Being lonely when alone is terrible but being lonely inside of a relationship is even worse. You have a constant reminder of how things used to be and what you hope they will one day return to. Instead of looking for someone ,like a single person,when you're in a relationship you're waiting for the other person to relieve the state of anxiety emotional neglect brings. That lonliness is mixed with feelings of inadequacy and insufficiency. "I'm not good enough" is often how rejection and emotional neglect are internalized. This leaves the person craving validation from their partner and when that craving goes unfulfilled it becomes easier to rationalize and internally justify cheating.
