Why does heartbreak hurt like physical pain?

Because, as far as brain scans can tell, losing someone you love uses some of the same machinery as physical pain. The overlap is real. But "heartbreak is the same as physical pain" goes further than the evidence does.

Here is the experiment that set people talking. Researchers scanned people who had been rejected by a partner about 63 days earlier, on average, while they looked at a photograph of the person who left them. They reported spending more than 85% of their waking hours thinking about that person. In the scanner, the brain's dopamine-linked reward areas were active, more strongly than in people who were happily in love, and so were insular and anterior cingulate areas that other studies link to physical pain. It looks like wanting turned into aching: people in love may even want to contact an ex when it will only hurt.

Why would a brain build it this way? MacDonald and Leary proposed in 2005 that the social-attachment system uses the pain system to push us to keep close relationships by hurting when we lose them. One telling detail comes from ball-toss experiments: people feel rejected even when they know the other players are only a computer.

Be careful with the neat version, though. The neurological process behind heartache is not known. Whether the painkiller acetaminophen helps with general psychological pain is disputed, and some researchers found that rejection can bring emotional numbness rather than more pain. The medical "broken heart syndrome", takotsubo, is real but a different thing: a temporary weakening of the heart muscle that usually improves within months.