Something Missing
Aside from fulfilling whatever is thought to be absent in a relationship, an additional reason why infidelity can occur is revenge. Sometimes as a method of evening the score for something you did, a partner will sometimes cheat. In other terms, cheating is usually a sign of other issues in your relationship, or with one of the people in the relationship where the concept of the others actions has caused a breakdown in the relationship. Not that having affairs is a remedy, more so than just a symptom of the underlying causes of affairs. Open honest interaction is very critical in a relationship and when you loose that, your relationship is headed in the wrong direction and exposed to huge danger.
We are extremely complicated creatures and our capacity to deceive other people is only surpassed by our capacity to fool our own selves. Not only do we have a tendency to lie to and manipulate others, but we compound our own problems (and the problems of others) by doing so. Even though this is not rational, it is brought about by being deceiptful with our own selves and could be caused by a huge variety of factors. The dilemma is, if a person cannot be truthful with themselves, who can they be sincere with? In this circumstance, honesty is missing and affairs are bound to eventually happen under these circumstances.