Science has come to few conclusions about the biology of itching. The itch reflex likely shares some of the same neural mechanisms and pathways as the pain reflex, but they’re obviously not the same thing. A nasty cut will make you pull your hand back, while a mosquito bite provokes an itch.
So why does scratching seem to help, at least temporarily? According to this 2003 New York Times piece (reprinted by the Milwaukee Journal Sentinel), the general theory is that scratching provides a “counterirritation” that distracts the brain from the original itch.
Others believe scratching releases pain-reducing endorphins. The pain neurons become temporarily overwhelmed, which masks the itching sensation.
But anyone who’s dealt with a bad batch of poison ivy knows scratching can often lead to more itching, which leads to more scratching, leading to all manner of pain, discomfort, and misery. We know this, and yet we continue to scratch because we are weak.
In short — leaves of three, let them be. Treat sun-damaged or dry skin with the appropriate ointments. Mosquito bites can be treated with topical antihistamines. And try not to scratch.