The thing I love about marriage is there seems to be someone for everyone. We're all different, and finding that person who gets us (or tolerates us) is a feat in itself.
Once we're married, the qualities that make each marriage great are as different as we are. However, there is one trait which can eat away at even the best marriages and ultimately destroy them. That is selfishness. Once you stop loving and serving other members of your family only to serve yourself and your own wants, things get ugly. You are no longer a part of a whole. You think you ARE the whole and you go searching for anything to keep you feeling that way. That leaves a spouse and possibly children wondering what they can do to make you love them and eventually everyone stops trying. The selfish person leaves, the ones left behind are broken forever - all because that one negative quality entered the picture. No matter what good qualities were in the marriage, selfishness pushed them out.
The best quality in a marriage...selflessness.