There are people who get into relationships with partners they see as far too good for them. Their lack of self-esteem can come from many different things, but they seldom raise the issue. They may be afraid their partner will suddenly realize they can do better, so panic overwhelms them. Rather than discussing the issue like an adult, they seek other ways to avoid it. Cheating on an unsuspecting partner leaves them feeling superior, but it will eventually destroy any love and respect they might have gained.
Childhood traumas or a lack of achievement in life are easy to hide even in the digital age, and they often have more to do with a person's inner dialogue than the reality the person they are with sees. A person might feel they are less intelligent than a partner, and it can cause them to believe they do not deserve the relationship. Destroying it by cheating is one way to accomplish the goal of destruction, but they then find that keeping the truth from the other person makes them feel more intelligent.
The continuation of that feeling of superiority is often what leads a spouse to keep going out and finding other partners. They may get little or no physical gratification from the act, and their goal could simply be to look at their partner and know they are smarter. While it may be years before a partner even suspects, the person cheating will go to great lengths to ensure their own bad behaviour is supporting their need to feel good.
Finding out a partner has continually cheated throughout a relationship is usually devastating, and the partner may have enough of their own confidence to blame the person for their bad behaviour. Citing reasons can be more of an exercise in bolstering their own internal feelings, yet the ultimate destruction of their relationship could leave them feeling worse than ever.