Speaking as an outsider, it's funny how some marriages work and others don't.
I know several people (my brother included) who've been married over 20 years and - although I'm sure they've had their ups and downs - there's never any question of the marriage ending.
Whereas I have other friends who've been married more than once and you feel that their current marriages/relationships are always balanced on a knife edge and could easily fall apart.
So there's something fundamentally different about those relationships, but it's really hard to work out what it is. Especially since they all went in with essentially the same intentions.