An Unexpected Revenge
One of the possible causes why some people enter an affair is because of past unresolved issues from their younger years, like an unrequited love perhaps. These unresolved issues might be in the past, but it can surely haunt you in the present when you least expect it. And trust, they could surely mess up the present.
A proof of this matter is the story shared by an anonymous user whose husband had an affair with a close friend from college. While they moved past it, she soon realized that her husband tried to change her appearance, which is somewhat similar to the appearance of the college friend. She left her husband and sought comfort from her best friend, who eventually fell for her.