Little is known about why we itch. It is a mystery after all. I suspect it is that infamous energy inside the body that roams up and down the DNA structure and labels itself as God (DIEU). This setting to revenge the human souls.
They certainly do not know why they created us, now they want us to know that they really exist but perhaps in a spooky way. Humans, on the other hand, do not care to know these things. When the revelation of the spirit comes upon one human it demands to do in a way that is pleasing to the world. This will avoid a trip to the mental institution. We, humankind, just want to Live, Laugh & Love.
Talking about itching all over the body. Sometimes we can see a cause of the itch, such as a scab or an insect bite. Stimulating pain receptors can cause itching, but no nerve endings have been specifically connected with itching alone.
For sure, thinking about an itch can bring on the urge to scratch, search no more, it is what has invaded me one night as I slept, many many years ago.