Fear of change and having no "deal breakers" can cover up a multitude of sins. It takes courage to and self-esteem to move on.
Nevertheless just because someone decides to stay does not mean they are "happy" nor will anything ever be "the same" again.
The mindset of a cheater is a selfish one. It has nothing to do with their overall feeling about their mate. In their mind if they did not "love you" they would have dumped you!
Most cheaters are looking to hold onto all that is good in their primary relationship while addressing their other "needs" whatever they might be on the side. The person they are cheating with is seen as a "compliment" to the relationship. Cheaters for the most part are not looking to replace one relationship with another. If their mate doesn't do something, neglects them, or simply takes them for granted they may cheat with someone who will give them (those) things.
Having said that there are different types of cheaters. The worst kind is the "incessant cheater". Their motto is "variety is the spice of life".
They most likely have never been faithful in a long-term relationship because they get BORED easily! They're usually very charming, enjoy being the center of attention, very flirtatious, and often use sexual innuendo. They're addicted to the excitement of being with someone "new". It's just a matter of (when) and not (if) they will cheat on you.
The "incessant cheater" does not need a "reason" to cheat. You can do (everything) this person asks of you and they are still going to cheat because (you can never become "new" again).
Cheating is their way of life!