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.
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.