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 if it adds nothing, then it should not be made. The curious thing is that lovers themselves, while they remain really in love, know this better than those who talk about love. As Chesterton pointed out, those who are in love have a natural inclination to bind themselves by promises. Love songs all over the world are full of vows of eternal constancy. The Christian law is not forcing upon the passion of love something which is foreign to that passion’s own nature: it is demanding that lovers should take seriously something which their passion of itself impels them to do.
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 me 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.
—from Mere Christianity