Rav Avigdor Miller on Why We Itch and Scratch
Q:
Why did Hashem create the sensation of itching?

A:
Most itching is a kosher sensation. People have forgotten that up until recently, human beings were infested with parasites. Even the queen of France, when she took off her big high wig, an expensive wig, she had inside the wig a little mouse trap. Kings and queens had servants who used to take the king’s head in their lap, or the queen’s head in their lap, and examine their heads. Human beings were troubled with parasites all the time.

Now, a parasite, if you ignore it, burrows into the skin. And once it burrows into the skin, it becomes a problem. So Hakodosh Boruch Hu created a reaction of itching, so that you should scratch. The purpose of that itching sensation was to immediately to catch him at his first bite when he starts mining into you, and to decapitate him immediately. Your nails would scratch and immediately the parasite is finished.

It’s a wonderful creation, this ability to have fingernails. We don’t realize how precious our fingernails are to us. Of course, if you ever try to untie a tough shoelace after you cut your fingernails short, you’ll see how difficult it is. You need fingernails for everything. But for this time honored practice of picking off cooties, fingernails are especially beautiful. A wonderful invention! The itching sensation and fingernails were a wonderful combination.

Now sometimes there is a sensation that’s an imitation of a parasite, it’s a false itching; then a person should refrain from scratching because then scratching will only irritate the skin and aggravate the situation.