If you think about it, marriage is a funny institution. Two young people decide they want to spend the rest of their lives together. This is a very momentous decision, of course, so they think long and hard about it. They consider all aspects of a person: their looks, their personality, their wit, their charm, their earning potential, their character, their goals and beliefs. Of course different people place emphasis on different things, but mostly the decision comes down to the fact that there is something about the other person's body, mind, or intellect that's appealing.
The irony here is that these people are making supposedly permanent decisions based on entirely temporary things. When they make their decision they're at the prime of their lives. They're young and fit and sharp and as attractive as they're ever going to be. From there they have some years of youth left and then, for most people, it's all downhill. The body sags, potbellies form, the intellect dulls, the mind gets set in annoying habits, moods become more irritable—in other words, their spouse no longer become the person they married. If they were to reevaluate their spouse 10 years later, they probably wouldn't marry them.
This may sound like a sad thing, but I think it gets at the essential beauty of marriage. They say a true friend is one who knows your faults and sticks around anyway. I think that's the point of marriage. When you grow old and no one else is willing to put up with your bodily noises and forgetfulness and all the other problems of aging, at least you can have your spouse; and, likewise, they have you.