Scientists and doctors do not reliably know the causes of itching
Nir Carlson Physiology of Behavior or Neuropathic Itch
Anne Louise Oaklander, MD, PhD,
For example ... The rest of the answers are either guesses or not generally accepted theories or tales from medical lectures
The feeling that something "itches" is a little pain. That is, if this same signal is strengthened, you will get a feeling of pain as from an abrasion. It turns out that when something itches, it hurts individual cells, not their entire colonies.
These are locally produced substances called bradykinins - mediators of itching (well, pain as well). Bradykinins are also formed during inflammation. So itching for no reason can be called a kind of underinflammation, when there is only a feeling of itching, but no pain, redness, edema, etc.