Well, why? Sometimes it’s due to a mosquito-bite event. Sometimes we just need a backscratch. What makes us itch? What makes it stop itching when we scratch? How come it sometimes returns worse than before?
Oh; one more (personal) itch-provoking event: when I eat spicy food, my scalp itches. Really.
(I don’t have a source to back up on, but…)
An itch is pain that is close to the threshold of pain. When you scratch you give yourself something bigger to worry about. Basically, you cover up a small localized pain with a lot of pressure.
I have no idea about spicy foods, but Chinese mustard makes the back of my head hurt, so I think I know what you’re talking about…
I don’t remember why we itch, but I’ve heard scratching temporarily damages the nerve endings to relieve us from the sensation.
Itch is a mild stimulation of the pain receptors (carried in the lateral spinothalamic tract).
The head itch stuff with spicy foods is related to why some people sweat with spicy foods (gustatory sweating) and has to do with less than precise wiring in some people’s crainial nerves (Facial and Trigeminal).
Itching and pain come in different contexts and generate different responses (the scratching impulse is obviously adaptive just for itching - shooing away any potential insect causing the itch e.g. - but maladaptive for most pains). A growing pain doesn’t start out as an itch either.
Pinch yourself slowly. At first you just feel your fingers, then you feel a little pressure then a little discomfort and then pain. No itching at any point.
So it can’t be a just a mild stimulation of pain receptors. Something else is going on.
Itching is frequently a reaction to a release of histamine in the body. That’s what’s happening when a healing scab starts to itch and, of course in allergic phenomenae.
Opiates also cause the body to release histamine, hence the theatrical depictions you see of junkies always scratching themselves. Even, endorphins (the so-called natural opiates) can cause you to release histamine. When I’m happy my nose itches.