“Being in love,” C.S. Lewis wrote in Mere Christianity, “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 just a feeling. Now no feeling can be relied on to last in its full intensity, or even to last at all…In fact, the state of being in love usually does not last…But of course ceasing to be ‘in love’ need not mean ceasing to love.
“Love…is a deep unity, maintained by the will and deliberately strengthened by habit; reinforced by the grace which both partners ask and receive from God…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 their promise. It is on this love that the engine of marriage is run: being in love was the explosion that started it.”