This principle - that your spouse should be capable of becoming your best friend - is a game changer when you address the question of compatibility in a prospective spouse. If you think of marriage largely in terms of erotic love, then compatibility means sexual chemistry and appeal. If you think of marriage largely as a way to move into the kind of social status in life you desire, then compatibility means being part of the desired social class, and perhaps common tastes and aspirations for lifestyle. The problem with these factors is that they are not durable. Physical attractiveness will wane, no matter how hard you work to delay its departure. And socio-economic status unfortunately can change almost overnight. When people think they have found compatibility based on these things, they often make the painful discovery that they have built their relationship on unstable ground. A woman 'lets herself go' or a man loses his job, and the compatibility foundation falls apart.