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 witch 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.
Mere Christianity, p . 109