A study published in the July issue of the Journal of Neurophysiology found that romantic break-ups activate parts of the brain that are associated with addiction cravings:

"This brain imaging study of individuals who were still 'in love' with their rejecter supplies further evidence that the passion of 'romantic love' is a goal-oriented motivation state rather than a specific emotion" the researchers concluded, noting that brain imaging showed some similarities between romantic rejection and cocaine craving. "The findings are consistent with the hypothesis that romantic love is a specific form of addiction." The study also helps to explain "why feelings and behaviors related to romantic rejection are difficult to control" and why extreme behaviors associated with romantic rejection such as stalking, homicide, suicide, and clinical depression occur in cultures all over the world, the researchers wrote.

I think most of us have experienced this feeling at one point in our lives, but it's interesting to know it can be backed up by science.

Anguish of romantic rejection may be linked to stimulation of areas of brain related to motivation, reward, and addiction [Science Daily]