Being in love is a good thing, but it is not the best thing. There are many things below it, but there are also things above it. You cannot make it the basis of a whole life. It is a noble feeling, but it is still a feeling. Now no feeling can be relied on to last in its full intensity, or even to last at all. Knowledge can last, principles can last, habits can last; but feelings come and go.
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 to never have a headache or always to feel hungry.