In one of the most painful studies on relationships, SUNY Stony Brook and Rutgers put students who had just been dumped through a psychological experiment that sounds worse than the breakup itself.
What they found was the same area of the brain associated with romantic rejection is also associated with craving, reward, motivation, physical pain, and addiction. Which explains the obsessive behavior after bad breakups: The late-night phone calls, drive-bys, cyber stalking, and other things you'd rather forget you did after a particularly painful heartbreak.
Here's how they found this out:
Each jilted student was hooked up to a functional magnetic resonance imaging (fMRI) scanner and asked to look at a picture of his or her ex (which -- oh, the indignity! -- the dumpees had to provide themselves). After looking at the photos, they were asked to count backward from 8,211 by 7s, then look at another picture of a person they knew but were not in love with, then count backward again. As if that exercise wasn't agonizing enough, the participants had to do it five more times.