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I read something on Wikipedia this is it "In some animal models, it has been shown that animals which excessively pull their hairs have more endorphin receptors in their brain than animals who do not. Endorphin receptors enable endorphins to have an effect on the brain. If this is true for humans, people who are particularly compulsive about their repetitive habits may have more endorphin receptors in their brain as well. This may explain why sufferers get more enjoyment out of picking their scabs than others." kind of makes sense.