From the other side, my ex husband began an affair with someone at work a few years ago. She was quite a bit younger than him, no children (we had 2), in a relationship but not married.
I think she made him feel young and fun again, whereas I was just the ageing, dependable wife and mum of his kids. He came home from work and I expected responsible adulthood from him and he went to work and could be fun and responsibility free.
Anyway, long story short, I found out and ended our marriage of 13 years. We had known each other since we were 16, lots of mutual couple friends, he was like a son to my parents.
I think reality hit quite quickly with the OW, that although it was fun being able to go out more and have a younger, fun girlfriend, she didn't understand his responsibilities towards his children who he still saw a lot, and the age gap meant that they wouldn't have had the same shared experiences. The novelty wore off and they split up after a year or so.
He is now single, after another failed relationship and I have been with my partner for 3 years and bought him out of the family home. He rents a house and I may be wrong but he doesn't seem like a happy man.
There was nothing wrong with our marriage other than the usual mundane-ness and predictability of married life with work and kids thrown in. I think he has realised that the grass wasn't greener.