The purpose of marriage is not and has ever been to be happy all the time. If you view love and marriage as about a transaction where the output must always be happiness, then you're setting an impossible standard. The author presupposed that marriage is about feeling really good all the time, well - most of us are going to hit hard times at some point. If you're so selfish as to believe your partner must always increase your own happiness - you're going to have a really bad time dealing with the realities in life. You might even be such a narcissist that you map your own failings onto the other person and decide they are 'wrong' when you're really a stubborn, selfish, unlovable hedonist.