I think there are some people (men and women but especially men) who love the thrill of the chase and the excitment of a new relationship. Probably also a power thing. They will cheat if they get the opportunity whether in a marriage or a long term relationship. They want their egos massaged, and it's a huge ego boost when someone takes an interest in you.
There are other people who are not interested in cheating, per se, think it is a shitty thing to do but, nevertheless, in the course of a long term relationship or marriage they come across one or possibly more people who they develop a strong connection with. It does not necessarily mean that their primary relationship is particularly flawed, it does not mean they are a horrible person. It just happens. Whether they chose to take that other relationship further (emotionally/physically) or even chose to leave the primary partner and move in with the other person depends on their own pysche and their beliefs about fidelity/marriage/monogamy.
Some people believe that serial monogamy is the most honest thing to do if you start to get bored in your primary relationship, and leave a trail of ex-wives, girlfriends and children without a full time parent behind. Other people believe that it is better to keep the family together and have a secondary relationship on the side (the old style mistress/lover scenario) on the basis that, in the end, the secondary relationship may well go the way of the primary relationship. Other people chose to stay in the relationship and do nothing at all about other people who have come into their lives as they could not deal with the guilt of a) splitting up a family b)upsetting the spouse and c) cheating.
I am in the latter camp. I am not a saint, in fact in some ways I am a coward. But I like a nice, quiet life without any drama or upset and do not want to be responsible for other people's distress, even if I myself feel a bit bored. Also, I can take the moral high ground which I wouldn't be able to do if I had indulged in a couple of flings, tempting as that was at the time.