There are a big number of theories floating around. I bet if anyone had a definitive answer, there wouldn't be so many divorces. Personally, I think it's just that the average American doesn't take marriage anywhere near as seriously as they should. It's supposed to be a lifetime commitment, and some people just don't seem to get that.
Heck, I have a cousin, barely out of high school, who wants to get married to some boy she's known for about a year. Sure, it's her choice and she's legally old enough to do it, but I and the rest of my family have doubts about this working out. Particularly since we think a major motivating factor is that she wants the distinction of getting married before her elder sisters (one of whom is marrying this summer).
Let me clarify: I'm not saying that married couples shouldn't be allowed to divorce. I'm saying that people should choose their marriage partners more carefully in the first place.