Surely not all not all the sensations of the human body (or any other body) are the outcome of evolutionary selection.

It doesn't seem surprising that the body experiences particular, unusual, sensations when new skin or scar tissue is being grown. Doesn't new tissue contain new nerves? At least there must be some metabolic mechanism that "connects" the newly grown tissue with the nervous system, so that you would feel a painful sensation if someone pinched your (freshly grown) scar tissue.

What sensation does the body experience when that "connection" begins to be made?

Maybe the metabolic processes by which the scab begins to separate from the wound when the healing is nearly complete (which is often the point at which the itching starts) produce the itching sensation.

Whatever the bio-mechanical origin of the itching, it is maybe just a side-effect of the repair process that serves no purpose of itself, but that is just part and parcel of a bodily repair system that obviously HAS evolved to high degree of sophistication to perform a task that confers major gene-survival advantages.

If scratching really does have shown negative effects (infection, slowing of tissue growth etc.) then evolution usually has the complexity to work around it. Even if while producing new skin it causes the itch sensation you would think the brain could easily compensate and "ignore" the urge to scratch it, it would be a simple level of instinct, yet my instinct nearly always tells me to SCRATCH.

It also leads to scarring, which while not directly affecting the persons health, it can be shown that it definately has an effect on mating patterns, levels of trust etc. A "scratcher" could end up being one very ugly pup.

How I make a beeline to so many stupid things said by people getting paid can only be the internet...

A wound is healing, healing is growth, growth is felt. To itch is the side effect. Not to mention the wound that is healing could have something undesirable within.

"If scratching really does have shown negative effects (infection, slowing of tissue growth etc.) then evolution usually has the complexity to work around it."

There again, evolution may not be all it's cracked up to be. Maybe itching - and consequent scratching - is just a ba-ad thing that doesn't help us at all. Maybe we'd all be much better off if we didn't do it. Maybe all those sabre-toothed tigers were even more itchy than we were. Maybe itchy wounds are caused by wolf genes "reaching out into the world" a la Dawkins to weaken their prey. In a world without references or personal biographies anything is possible.

I thought the itching was just the bits of torn tissue beginning to join up again.

Well, most animals do not scratch wounds - they lick them. The itching reminds them to keep the wound clean and moist.

Maybe the wounds just taste good. Maybe it's the salt in the blood. Check it out - lick your wounds!

So how come these animals can overcome the scratching urge like that? Maybe they don't itch? There again, my dog scatched her eczema to shreds and then licked the wounds.

Maybe there's evolutionary value in not knowing these things?