The idea that "being in love" is the only reason for remaining married really leaves no room for marriage as a contract or promise at all. If love is the whole thing, then the promise can add nothing. . . and, of course, the promise, made when I am in love and because I am in love, to be true to the beloved as long as I live, commits one to being true even if I cease to be in love. A promise must be about things that I can do, about actions: no one can promise to go on feeling in a certain way. He might as well promise never to have a headache or always to feel hungry. . . .
Being in love is a good thing, but it is not the best thing. You cannot make it the basis of a whole life. It is a noble feeling, but it is still a feeling. Now no feeling can be relied on to last in its full intensity, or even to last at all. And in fact, whatever people say, the state called "being in love" usually does not last. If the old fairy-tale ending "They lived happily ever after" is taken to mean "They felt for the next fifty years exactly as they felt the day before they were married," then it says what probably never was nor ever could be true, and would be highly undesirable if it were. But of course, ceasing to be "in love" need not mean ceasing to love. Love in this second sense is not merely a feeling. It is a deep unity, maintained by the will and deliberately strengthened by habit. Being in love first moved them to promise fidelity; this quieter love enables them to keep the promise. (from Mere Christianity)