If you ask me (married 23 years), it's because people get married for the wrong reasons. They get married because society expects them to get married, nobody tells them that spending a lot of time with another human being is really, really hard (in this modern world where both partners are equal partners with full rights), and when - not if - things go wrong, they think "the marriage" is falling apart. So it does.
My wife and I got married after 6 weeks of acquaintance, in order to get visas that would allow us to live in the same country so we could get to know each other better. The deal was, if it didn't work, it didn't work, and we'd go our separate ways.
But once you've made that commitment, and you realize this person is really somebody you like, a lot, and you'd have to research international law to figure out a divorce, then ... well, you learn to work through the most unbelievable and outrageous problems.
We have had very, very bad times. And we've had a lot of pretty damn good times. Every year, we don't get divorced.
There's nothing magical about marriage. There's most definitely nothing magical about love. You can love somebody passionately and that has nothing at all to do with marriage. Confusing these things is why people hurt themselves so often.
Just to add to this, which I agree with, I also think conflict resolution is huge.
You really gotta get that bad stuff out in the open, talk about it, yell about, whatever it takes. Don't be pissed off about something for weeks on end (even more than a day is too long). To keep a marriage strong, both partners have to learn to deal w/ each other's idiosyncrasies and communicate about perceived grievances, annoyances, and so on. Extreme stubbornness, selfishness, all these things are pretty much incompatible with a healthy marriage.
This is exactly it. I've only been married 4 years, so I'm only speaking with a little experience, but when the love, passion, lust, etc. disappears, what is left? In the end, commitment is all that keeps it together. We happen to both be committed, and both have good examples in our parents (neither divorced, which is rare). We now have children, which helps us keep the proper focus. In the end, keeping a marriage together has more to do with being able to put your personal interests aside for large chunks of time (not all the time, just sometimes). As you said, the bad times can be bad, but the good times can equally be good. If we weren't both equally committed, I can see how things can unravel. Sometimes the fault is with both parties, often it's because one person is not as committed. The key is to find someone that is equal in all that.