Itch

Why do we get itchy skin?
I can understand the need for a sensory system which picks up warnings essential for safekeeping of the body.
Temperature to warn against burns.
Pain when a sharp object is pushed through the skin.
Pressure, so we can avoid blisters and breaks
Movement, when the hair on your arms moves in a breeze, or your semicircular canals pick up 3D movement of the head.
But why an itch.

I can understand if it were a warning of a bug bite which might get infected or worse, but sometimes nothing seems to cause it.
You're just sitting minding your own business, when suddenly your scalp itches, or your elbow itches, or the back of your neck, your waist, your nose, your ear starts to itch. No obvious cause.
And why does scratching stop it? I know that physiologically and biochemically there's a cascade release of serotonins and natural histamines and antihistamines, but why is it so satisfying?
At the right time, at the right place, a really good scratch approaches pleasure levels of orgasm.
You laugh?

Think. You've had an itchy arse for at least 40 minutes. You cannot scratch it because you're doing something you cannot stop, in a public place. Teaching a class for example.
You try to ignore the irritability which ignoring an itch causes.
The class leaves, you shut the door, and flatten against the wall so nobody can see you through the window in the door.
You start to scratch. Getting to the point of itchiness is awkward, seeing where the itch is based, but you manage.
Ahhhhh, the relief, you close your eyes. More scratching gives even more relief, and waves of satisfaction emanate from the contact point.
Careful, you're approaching the point of maximum relief and the onset of pain caused by abrasion, the dreaded over-scratch.
You stop, making a mental note to wash your hands before eating or drinking.
Pushing away from the wall, to get back to your desk, you notice that through the great big bloody window between your room and the next 4 grinning and smirking kids faces, giggling and pointing at you.
Your reputation as some sort of perve is now set in stone.
Was it worth it?

Ahhh yes it was.