Okay, I have a theory. I believe falling in love is an addiction. First, there's the feel-good, the immense buzz. You don't believe people when they say you're addicted. You act like a fool. Then, you get tolerant, and need more and more love to get the same buzz. If the love-object is taken away, you start getting withdrawal symptoms. There is craving, there are physical and mental symptoms of withdrawal, anxiety and depression. You look for substitutes to ease the pain, and they are often more harmful than the original. If you manage to survive, then slowly, the craving goes away. Yet, even after years, a sensory trigger may set off the cravings.
There must be neurons in the brain that are fired by love, just like there are receptors for drugs and reward pathways.
And since there is tolerance, withdrawal, and significant impairment in social functioning, it is a dependence according to the DSM IV criteria.
I know its not the most original theory in the Universe, but still. This needs to get into DSM V.