I answered this question on Quora, and thought it was worthwhile posting the answer here, as well.
Q: Is it easier to get over someone you cared for once by enthusiastically hating them?
Actually, while hating is not ultimately a rewarding or useful emotion, in the short term it could be a practical way to reframe the physiological state of broken-heartedness and longing for a lost love. Hate is related to love as a state in the body and brain.
Unfortunately, advice to just forget about him or her doesn't work well. It's difficult to force emotions to change, and it's not emotionally healthy. When you tamp down any emotion, even a negative one, you are reducing your ability to fully feel all emotions.
FMRI studies have shown that the state of romantic love is similar to reward-seeking and competitive activities, which usually are characterized by high levels of dopamine. (Dopamine is the brain chemical of pleasure, but levels begin to drop as soon as you get the reward.) It's likely that the brain chemistry of the person dumped is also high in testosterone (lusting for the beloved) and oxytocin (being bonded to the beloved).
Now, here's the speculative connection between love and hate:
A couple of recent studies have shown that oxytocin also plays a part in aggression toward outsiders; testosterone is also a chemical of aggression; and dopamine creates high focus on its object.
Biologists define emotion as a physiological state that we then name using the prefrontal cortex. So, if our broken-hearted person already has lots of oxytocin, testosterone and dopamine circulating in his or her brain, creating these responses, it should not be too difficult to reframe the emotion as hate. "He's awful. I wouldn't want her."
If you remove the reward-seeking element, dopamine levels should drop, allowing the focus on the other person to fade away.