I think there is lack of wooing often on both sides. Something else I've noticed. I've seen couples who live together 5, 10, 20 years decide to tie the knot to become honest couples. 6 months to a year later they divorce. Can't get along. Why could they do it before the ring and not after?
I dunno. Maybe they expected the marriage to fix what was wrong, but didn't let it change them enough. Or maybe it changed one of them and not the other.