Why do itches move?

Do you ever have an itch that just won't be scratched?
Many a night I have a itch on my back that itches with an intensity that would drive you mad if it wasn't scratched straight away. Of course the itch is just out of reach and I have to ask Andy to scratch it. Andy seems to takes his time and some times I sure on purpose, pretending not to know where the itch is.
I could do with some kind of grid tattooed on my back, so I can say go straight to G5 and when it move I can give more direction A7-F2-B9 and so on.
But what makes the itching sensation and why does the itch move? I have had a look on the web all that I can find is a skin irritation eg: allergy, rash, lose skin extra will cause the itch, well nothing we don't already know.
This still hasn't answered my question, why a itch moves. So if any one has a idea I would be intrsted to know.