Problems aren't mandatory - but with one in three marriages and probably as committed relationships failing - well you're really pretty stupid, if you can't see the odds ain't great for success. That's not counting all the relationships where they stay together, but its miserable as sin for everyone involved.

If you've got a happy relationship - don't talk too loud and proud. As long as you're both above ground, the wheel is still in spin. Don't take anything you've got for granted - we're all from the same clay. Its so easy to end up in a room with the two of you swimming through a mass of home truths, and as TS Eliot remarked, humankind cannot bear very much reality.