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 parents 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.
–From “Mere Christianity” by C.S. Lewis