For those who have problem with their back pain!
Back scratching for tingles and relaxation. LONG NAILS!!!
It is thought that scratching releases endorphins, which are pleasurable chemicals created by the body in response to a number of stimuli. In the case of itch-scratching, the temporary relief of symptoms may result in the comparatively significant signal that scratching sends to the brain. Scratching is a clearer stimulus than itching, so the brain pays attention to it rather than the persistent itch.
Since most people cannot scratch their own back (and even those that can have to bend their body in awkward ways), the help of another is much appreciated and results in a more efficient scratch.