The New York Times has an interesting summary of our current understanding of itching and scratching.
My own understanding is that some itches are God’s way of encouraging us to be nice to each other. I refer to that perpetually itchy spot in the upper middle of our backs, the one that only contortionists can scratch. Is there any other reason it should be so itchy? I can’t see one; and so I believe it itches so our loved ones can scratch it for us, to our intense gratification and delight.
Note: God isn’t strictly necessary for this conjecture to work. You could also say that the itchy spot between our shoulderblades is evolution’s way of reminding us that we’re happier when we cooperate.