Every situation/relationship is different just as every person is different. Nonetheless the person cheating has a "need" of some kind. Needs are the motivation for all human behavior. (We tend to do what we feel we need to do in order to feel good).
Having said that there are 3 basic types of cheaters.
Keep in mind most cheaters don't plan on getting caught which explains why they would consider cheating to be a better option than going through an expensive or messy divorce/breakup. A cheater seeks to hold onto what is good/comfortable in his or her primary relationship while addressing their other needs on the side.
1. The Incessant Cheater
This individual has no intention of honoring any vow or promise they make. He or she is always looking for the next thrill or something "new". Most likely this person has never had a monogamus relationship in their life. They usually flirt a lot and people are easily drawn to them. Their motto is "Variety is the spice of life."
2. The Unbelievable Opportunity Cheater
This person actually intended to be faithful and is not "looking " to betray their partner. They give into temptation when an "unbelievable opportunity" presents itself.
An extremely attractive person who shows interest in them, a girl or guy they had a crush on in school comes to town and wants to "hang out" and last but not least a famous celebrity or politically powerful person offers what appears to be "A once in a lifetime opportunity" to have sex with someone out of their league.
Their motto is :"The only way to get rid of temptation is to yield to it"
3. The Discontented Cheater
This person believed they found their soul mate and never dreamed they'd be considering cheating. Unlike The Incessant and The Unbelievable Opportunity Cheater, The Discontented Cheater feels as though his/her partner is (forcing them) to "seek other options" outside of their relationship. It's not a stretch for them to feel “justified” in seeking other ways to fill a need that is not being met. Depression often serves as motivation to take a risk. It's also not uncommon for them to not be looking to cheat. However it's not difficult for a stranger to put a smile on the face and in the heart of a discontented person.