What is an itch, and why are most so easily calmed with a simple touch?
Your questioner clearly hasn’t lost many nights’ sleep being driven crazy by, say, a mosquito bite on the back of a toe, seemingly refractory to all treatments other than scratching it until it hurts, which feels better than the itch. Maybe itching has evolved to alert us to attack by blood-sucking arthropods, and if the persistence of the itch leads to a visceral loathing thereof, then surely that too, given dangers like the plague or malaria, will confer an evolutionary survival advantage.
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