All of the examples and reasons for divorce, with the exception of one or two have directly been a result of one party acting in a way that favors himself/herself over the other. You can call it selfishness, with all of its negative connotations, but that's what it is. Adultery, poor communication, finances, hedonism in whatever form, uncontrolled habits (drugs, food, spending, etc.), are all forms of selfishness. Sure, there are examples of mental illness as previously quoted, but those are fewer and farther between, and are more representative of the minority than the majority.
One funny observation I've made: Divorce is almost always the other person's fault. Listen to some divorced folks - their stories have a lot in common - it was the fault of the other person. My father blames my mother, my mother blames my father. To use a colloquial expression, it "takes two to tango". A team must be united in all things or it won't win. If the wide receiver wants to play a different play than the quarterback, there's a decreased chance of success on that play.