This is an interesting question. I think there are a variety of reasons - and dopamine is just one aspect.
I know the common refrain on here is 'once a cheater, always a cheater' etc, I'm not sure that's always the case.
I know a few people who are in successful long-term relationships after an affair. Many years ago, my dad left my mum after a brief affair. Although it caused turmoil and upset at the time, mum admitted herself they probably should never have been married. She entered the marriage with so much historical trauma that my dad couldn't fix. Although I think they loved each other at the time, they were young, and they couldn't meet each other's needs in the relationship. My dad has subsequently been in a committed relationship for around 20 years now - longer that with my mum. They are good together, and his LP, who is great, credits my father for breaking her history of picking terrible partners.
Another male friend was is a long term relationship with an utterly toxic partner. He ended up (after many unhappy years) having an affair with a woman in our wider friendship group. Most of us had no idea, but it was clearly an exit affair. He is now married to her and very committed. I never got the impression he was some sort of serial cheat.
I know this isn't always the case - there are plenty of a-holes out there, and if you've had your world torn apart by one of those it must be terrible. But there are plenty of people out there who have an affair because they are deeply unhappy, and then once this unhappiness is resolved, they don't necessarily go searching for that excitement again - whether for dopmanine or anything else.