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.