I believe one realizes their marriage is falling apart when he or she no longer wants the same things as their spouse. No amount of "communication" or "work" can overcome being with someone who does not want what you want.
A successful marriage is a (team) effort. Opposites may attract in the short run but it's like that attracts like in the long run. Ultimately we are all looking for someone who (naturally agrees) with us on the major things in life. A couple has to want the same things and have the same goals for the marriage as well as (agree) on how to obtain them.
Couples who agree with each other most of the time tend to stay together. Who knew?! :-)