A gripe that is familiar to many: "Our relationship was great until we got married."
Why does getting married change relationships for the worse?
During each of the steps leading up to marriage - traveling together, meeting the family, living together - you are on your best behavior. You're doing all those appealing things with the goal of being deemed a partner worthy of marriage. Then you get married, so you are no longer aiming for marriage. You cease to be on your best behavior; wearing your best clothes, going to the gym, going the extra mile in every way. You have crossed the finish line, so you stop running.
Before you tie the knot, the relationship benefits from constant oneupsmanship. Your partner jogs, so you start jogging. Your partner is generous in bed, so you try to be generous in bed. You are constantly trying to keep up with your partner so that she will remain interested. It is a romantic arms race...a bit like the Cold War, only in this case the end result IS World War III.
Once you're married, the tendency is to rest on your laurels, because in theory, marriage is permanent and you're no longer competing with half the species to win your partner's heart. You think you have a monopoly on her love, and predictably, you become a worse product. You let your hair down, and in doing so, let your spouse down.
Now with divorce being so common and cheating discussed so openly, maybe people will wake up to the fact that they absolutely must try to excel in every way, or else their ladyfriend will hastily find someone who hasn't yet achieved all his goals with her. It could be that the answer to divorce is high divorce rates.
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