You scratch an itch to cause pain, which soothes the itch. Then your body releases serotonin to reduce the pain, and the itch flares up.
The result helps explain why people can get hooked on scratching an itch to the point that they bleed, says Chen. "People scratch because the pain inhibits the itch," he says – but because serotonin controls the pain, they then have to scratch harder to register pain strong enough to suppress the itch, creating a feedback loop. "The brain wants to control the pain so it puts out more and more serotonin."
