The state of a marriage is not set in stone. It is absolutely possible (and happens all the time) that a great marriage falls apart because one or both partners gradually stop giving attention to each other.
The opposite may also be the case. A marriage that started off badly or took a big hit still can be amended.
It just doesn't happen automatically.
Marriage is mainly what both partners make of it, und requires constant work and care. Of all the divorces I have seen in my environment, there was always at least one partner who wasn't willing to give his share of commitment.
Quote I heard somewhere:
After 65 years of marriage, an old couple was asked how they managed to stay together for so long. The woman thought for a few seconds, and then replied: "You know, we were born in a time where people used to repair things instead of throwing them away."
 This admittedly does mean that the fate of the marriage doesn't lie completely in the hands of one individual. You can't single-handedly make your marriage a good one. So maybe thats even the singlemost important trait to look for in a potential partner: Check for the ability to cooperate.