« —One may promise actions, but no sentiments, for these are involuntary. Whoever promises to love or hate a person, or be faithful to him for ever, promises something which is not within his power; he can certainly promise such actions as are usually the results of love, hate, or fidelity, but which may also spring from other motives ; for many ways and motives lead to one and the same action. The promise to love some one for ever is, therefore, really : So long as I love you I will act towards you in a loving way ; if I cease to love you, you will still receive the same treatment from me, although inspired by other motives, so that our fellow-men will still be deluded into the belief that our love is unchanged and ever the same. One promises therefore, the continuation of the semblance of love, when, without self-deception, one speaks vows of eternal love.»
– Nietzsche, in. Human, All Too Human, second division, aphorism 58.
Sums this great oneshot pretty well.