It is strange how when you've had the "light bulb" moment, you can't understand why others can't see the light too!
My ex-H had an affair 11 years ago. At first I was livid and full of anger - how could you do this? How could you break the promises? How could you deceive me? etc.
Then we went to counselling in an attempt to see if we could find a way forward. The counselling was all about my ex-H. All about how hard he found having young children, how hard he found sharing my attention, how lonely he sometimes felt, how he no longer felt I was interested in him, how I didn't understand him anymore. I sat through week after week after week of this shit and I started to doubt myself. I started to think that somehow it was my fault that ex-H was so miserable that I had somehow pushed him to the point of having an affair. I'm a control freak, a pleaser, a fixer & he is a narcissist. I started to think I could fix him & make it all better.
I tried, he didn't. We got divorced.
However, it took a decade to undo that mind set that I had somehow unwittingly, unknowingly made ex-H be unfaithful to me. That having babies (that he wanted more than I had done) and not being able to give 100% of myself to him meant that I had forced him to the point of an affair.
Having an affair is a selfish choice that one person in a marriage makes. It is not because of the problems, it is because that is the course of action they choose. Other people might choose to improve communications, to go to counselling, to get away together & rediscover themselves as a couple, to help the partner with drink issues, eating issues or MH issues etc etc, but the partner who has an affair makes a conscious choice to do lie, deceive & be unfaithful, rather than leave or try to sort out the problems.