Why we scratch
Why do we scratch?
A simple answer would be because we itch, but how does scratching make itching (or pruritis in doctor lingo) better. But the real answer is
the most characteristic response to itching is the scratch reflex: a more or less voluntary, often subconscious motoric activity to counteract the itch by slightly painful stimuli. This itch reduction is based on a spinal antagonism between pain- and itch-processing neuronsIn other words scratching causes a mild, painful sensation that inhibits transmission of the "itch" signal:
I never knew that!
Evolutionarily this makes sense: noxious (e.g. painful stimuli) cause withdrawl, but itching usually signifies something on or under the skin. The pain from scratching prompts closer inspection.
The linked article has everything you'd ever want to know about itching and more.