Love, in the Christian sense, does not mean an emotion. It is a state not of the feelings but of the will; that state of the will which we have naturally about ourselves, and must learn to have about other people.
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.