IMO, the major things are mismatched trajectories (one partner wants to live/work elsewhere), deal breakers that weren't discussed early enough in the relationship (kids/no kids, save vs spend, religion), and major personality differences or lack of common interests that were ignorable during the honeymoon phase but become a point of resentment over time. That last one being a big one.

I hear a lot of men in my office complain about their wives, but I think a lot of their complaints stem from the fact that they have little to nothing in common with their wives. They complain about things like not liking the same type of people as friends, not wanting to watch the same movies, etc. But people who I've spoken to who have generally good relationships usually enjoy 90% of the same types of movies, books, activities, etc., and any differences are usually minor (occasional trash TV, maybe one partner likes one to two games/shows/activities that the other doesn't, etc.)

One explanation I can think of is that they say it takes around 4 years before you truly know someone, but many people get married after 1 or 2 years of dating. I think such a short dating period does not expose huge differences of thought enough that you realize that you are not compatible. Thoughts?

Edit: So it looks like most people agree that it's common values, not common interests that hold relationships together. I can agree to that.