An article in the NYT takes a look at how the brain reacts to new relationships. It also explain why breakups suck so much:
"Researchers have found that, among other processes, new love involves psychologically internalizing a lover, absorbing elements of the other person's opinions, hobbies, expressions, character, as well as sharing one's own. "The expansion of the self happens very rapidly, it's one of the most exhilarating experiences there is, and short of threatening our survival it is one thing that most motivates us," said Dr. Aron, of SUNY, a co-author of the study.
To lose all that, all at once, while still in love, plays havoc with the emotional, cognitive and deeper reward-driven areas of the brain. But the heightened activity in these areas inevitably settles down. And the circuits in the brain related to passion remain intact, the researchers say - intact and capable in time of flaring to life with someone new."