Neitzsche wrote in On the Genealogy of Morals that to make a promise is to make a prediction regarding one’s own future. To make a promise to someone else, if we follow that idea from Nietzsche, is to make a prediction regarding the couple’s entangled future. When that promise is broken, not only is it a betrayal of the trust it founded, but it also nullifies that future. It prevents it from ever coming to fruition.
I believe this is why breakups hurt the way they do. The person doing the leaving had promised something to the person being left. He promised her a future, and now he is taking that away after he made her wait for it. She planned for it, prepared for it, altered the course of her life for it. And when he recants, he undoes all of that hope. He takes her future away. How swiftly he can destroy a life she built but did not yet have the chance to live.