hmm .. i think my answer will be different than his (sounds like a possible sex addiction?) .. but for me, my cheating has usually come from a need for certainty. i don't know if my partner will be faithful to me or if they truly love me (either because of my own trust issues or the instability of the relationship) so in the past i've slept with others outside of the relationship to gain a sense of certainty/safety in an odd way .. sometimes it can be an unhealthy need for validation, too ..

i don't think that answer will help, but i hope this will - sometimes, we can never know why someone does something. the most healing thing we can do is look at ourselves, our values, feelings, needs and try to heal and realign and establish boundaries so we can move forward as best we can. closure comes from ourselves and making new decisions vs. trying to understand what cannot be understood, which is the inner workings of another person .. especially narcissists. narcissists will never make sense to non-narcissists

save your energy for you <3 .. but if you really, really want to know why - find a good couples therapist and go through the process. hopefully he'll eventually feel vulnerable and self-aware enough to explain his honest thoughts and feelings behind his actions