Therapist here: I'll tell you all the reasons I hear about in therapy for cheating and infidelity. No judgement right or wrong on my end- just the reasons given.
-I'm miserable but if I leave I can't support myself/my kids so I'm going to try and find happiness in brief moments with someone else.
-I love him/her but I'm not getting my sexual needs met. I love being with the person but they're not open to exploring other sexual acts or they don't please me sexually and I feel trapped.
-We're both unhappy but we can't afford to leave one another, so we're both cheating without talking about it.
-My partner doesn't care about my emotional needs, and I'm getting those needs met through others. The sex is a byproduct of a better emotional connection.
-I'm expected to stay with my husband/wife because that's what you do when you're married. My parents never divorced and neither should I.
-My partner has severe health/mental health issues. But I can't abandon/leave them but I don't love them anymore. So cheating helps me not feel so alone.