From Mere Christianity by C. S. Lewis:
Being in love is a good thing, but it is not the best thing. There are many things below it, but there are also many things above it. 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. Knowledge can last, principles can last, habits can last; but feelings come and go. 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 was never was or ever could be true, and would be highly undesirable if it were. Who could bear to live in that excitement for even five years? What would become of your work, your appetite, your sleep, your friendships? But, of course, ceasing to be “in love” need not mean ceasing to love. Love in this second sense — love as distinct from “being in love” is not merely a feeling. It is a deep unity, maintained by the will and deliberately strengthened by habit; reinforced by (in Christian marriages) the grace which both partners ask, and receive, from God. They can have this love for each other even at those moments when they do not like each other; as you love yourself even when you do not like yourself. They can retain this love even when each would easily, if they allowed themselves, be “in love” with someone else. “Being in love” first moved them to promise fidelity: this quieter love enables them to keep the promise. It is on this love that the engine of marriage is run: being in love was the explosion that started it.