It's an ego boost, attention, flattery, being put first, feeling special. It's having something secret because you deserve it. It's an illicit little bubble that takes you out of everyday life, and it won't hurt anyone because you'll stop it before anyone finds out. It's never being told you've had too much to drink and have you put the bins out.
This. This is exactly what my DH would and has said after a year of therapy. Even his therapist told him it didn't have much, if anything, to do with me or the quality of our marriage!
Another one here with the guy who would never do anything like that. I lost count of the number of people who told me that ours was the last relationship they would have suspected would have something like cheating happen in it. My husband adored me when we got married. We had regular, non vanilla sex several times a week, I am a size 8-10, a good cook, a professional. I think I am a nice person. What I am not: 27 and new.
I think the point above re just having to forget our fears and try to enjoy the relationship and be good partners ourselves is right. Just like anything in life, happiness and letting go comes with risk. I don't think I'll ever fully trust again, my DH or anyone else. That may sound a little sad, but you know I think it actually reflects reality better than my old view.
I get a little impatient too with those that say never would their partners do what mine did. It does strike me as rather arrogant (I'm special/my relationship is special!), but I'm willing to cut people some slack and wish them well. I'm not sure scaring people into being paranoid serves anyone well.