On what we can promise, Nietzsche stirs our thoughts with this beautiful passage:

One can promise actions, but not feelings, for the latter are involuntary. He who promises to love forever or hate forever or be forever faithful to someone is promising something that is not in his power. He can, however, promise those actions that are usually the consequence of love, hatred, or faithfulness, but that can also spring from other motives: for there are several paths and motives to an action. A promise to love someone forever, then, means, ‘As long as I love you I will render unto you the actions of love; if I no longer love you, you will continue to receive the same actions from me, if for other motives.’ Thus the illusion remains in the minds of one’s fellow men that the love is unchanged and still the same.

One is promising that the semblance of love will endure, then, when without self-deception one vows everlasting love.