Why Do Itches Itch?

As someone who has scratched plenty of itches, I can say this rings true to me. Originally I heard that itches were the body's way of telling its owner that some top-layer skin needs to come off (which may be partially true) but not all itches are created equal either, so there has to be plenty of brain activity going on with scratching as well. With athlete's foot, for example, I could scratch an area until it's bleeding without feeling anything but ecstasy (until the next day of course) whereas scratching is both painful and damaging as a hemorrhoids treatment (http://www.dermaremedies.com/hemorrhoids-removal-hemroids-treatment.html) from the first touch. Scalp itches tend to be in the middle, as they seem to go away almost immediately with a good scrape or 2.