What I've seen over my lifetime and perhaps the way I rationalize and explain it away is an ebb and flow in relationships. Everyone, males and females alike are constantly searching and trading out on the highs of a relationship, not sticking around for the lows and working thru those problems. This would explain high divorce rates to a large extent. I've seen the miserably unhappy woman and man in each situation, whether it be the same relationship/couple or one's that were in completely separate relationships altogether with others. I also have talked with women who want to be the trophy wife, the one's that shops all day and every day, travels back and forth, maybe even to exotic locations and so on. That's usually a short marriage/relationship, usually ending in replacement with yet another trophy.
As BD states above, men and women are entirely different creatures and that accounts for a slice of what goes on, but it is also their similarities that creates these rifts/failures in relationships, even the successful ones thrive and morph based upon these differences and similarities, it's just that the more successful relationships, the couple stuck around to see it thru, thru thick and thin. There are relationships that are built to last, dynasties and then there are those built to run short term. Like investing, you have your S&P 1,000 and then at the other end, the penny stocks of relationships. Sometimes I wonder how much of this is really over the similarities that either wasn't honest with themselves or their partners about going into that relationship. Dating is very much like a job interview, careers these days are far shorter with the same organizations, what are the goals of the parties ? Same holds for marriages and relationships, are each only looking to stay with each other to find yet a better deal. Both must be honest and genuine with each other on that end of it.