In my H's case, the o.w. provided him with the chance to recreate himself the way he wanted to be seen, and how he doesn't actually feel about himself. He told me he lied a lot about himself to her. He is actually a great man who has accomplished a LOT in life, been many places, etc. but he still doesn't feel it inside himself. o.w. hears all his stories for the first time, is impressed, and thus H. feels wonderful about himself. I was saying on flaneur's thread that it didn't matter that I already thought he was wonderful...he needed to hear it from someone new, someone who didn't know him before he'd accomplished so much. I think he feels I know the "real" him; o.w. knows the man he wants to be seen as.
Most of us really marry only once. First love endures, even unto our dying day. And we never really divorce.