Ugh, there's nothing quite like the relief felt when you scratch an itch, but why should scratching relieve it in the first place? A study published in the journal Nature Neuroscience found that itches don't actually occur at skin level but deep in the nervous system (this is sometimes why you can get a "referred" itch when the itching sensation is felt in a different place to the stimulus). They found that the scratching sensation interferes with these nerve signals and essentially tells them to calm down. This is interpreted as a sense of relief in the brain. This is a similar principle to pain management, in the sense that rubbing a sore knee will make it feel better because one sensation blocks the other. However, the principles of pain management also throw a bit of a spanner in the works and could actually make itching worse. The scratching sensation is essentially interpreted as a pain signal in the brain. Pain, generally speaking is a Bad Thing as far as the brain is concerned and so it does its best to soothe the sensation by releasing serotonin. Serotonin suppresses the "scratch" signal and can also, unfortunately, boost the "itch" signal, making the whole thing worse and producing an itch-scratch cycle that can be tough to break.