Because we are letting go of something (someone). Sometimes, you have to let go of a dream you had would come true with that person. And we are emotionally connected with them and when we break up, the connection is broken. It should be called mourning.

If it is difficult for you to leave someone, that means you're not ready. If you suffer, that usually means it wasn't your idea to break up. When a relationship has run its course, then (for me) it is hard to stay together and easy to leave. I suffer only if I stay and am relieved when it is finally over. I hope you reach the stage where you realize it is over and can move on, Elinor. Happy Wednesday! :)

everyone has a motivation, but mostly is denial. desire is my answer. if it breaks up something is not right. people forgot their innermost personal nature and hide it for the sake of fitting in this unnatural political/artificial/constricted world some people made. to fit, you have to repress your innermost nature, your sponataneous nature. when a relationship brokes up, is just because, in repressing our nature, we tend to deviate from our naturally ideal partner, and being deviated you mod yourself getting along with someone wich isn't your natural best. but sooner or later this is gonna showup. If something breaks, it was already broken before starting, you just couldn't listen at what your innermost nature was telling you. you found immediate pleasure in something, or best, you modded yourself in finding that, but being unnatural it leaked resources until the water lowered enough to discover all those foundamental problems wich were there already from the beginning. so, in the end, the answer is always 'wrong personal choice'. and wrong personal choice is always led by desire. desire is always led by deviated attitudes from your natural inclinations and this is due part to society and part to yourself, wich get deviated somehow. now i may be wrong, that's a my partial opinion