The main reasons I’ve seen are:
He’s just a miserable person all around, would be miserable single, in a relationship, married. He’s just unhappy.
Loved the wife initially when she was child free and not stressed out now that she has children, job, home etc she’s not as fun to be around.
He just doesn’t want to be married.
He doesn’t want to be with her anymore.
He didn’t want kids or didn’t want as many kids as they have and now he hates life.
His wife is cheating and he’s scared to leave because of money.
Miserable because of work/family/children, the marriage has nothing to do with it.