A nice synopsis of the root causes of divorce at EconLog. I found the following most interesting, but you should head over and check out the entire post:
3. Defection due to expected divergence in mate value. As evolutionary psychologists will tell you, female mate value peaks and starts to decline at a much earlier age than male mate value: It's a lot easier for a 45-year-old man to remarry than a 45-year-old woman. This creates a big incentive for men to promise lifetime fidelity, then jump ship.
4. Defection due to unexpected non-culpable divergence in mate value. Remember the part of the contract that says "for richer or poorer, in sickness and in health"? When one partner experiences an unexpected rise in mate value (e.g. one becomes a successful novelist) or experiences an unexpected fall (e.g. develops a horrible disease), one of the parties has a temptation to back out - and some do.
6. Punishment for unexpected and culpable decline in mate value - or in plain English, "You let yourself go!" The marriage contract may not explicitly say that you can't become a bum or morbidly obese or perpetually bitter. But you've heard about incomplete contracts, right? When one party falls far short of expected mate value due to deliberate action or inaction, divorce is not only likely, but easy for neutral outside observers to understand.
I especially like the incentive/economics angle in the above root causes.