Why does your body give you a good sensation when you scratch something you're not supposed to?

Scratching a scab may cause an infection. Many rashes itch because of allergic reactions. Poison ivy is impossible to not scratch without applying some lotion or aloe.

“The culprit behind the rash is a chemical in the sap of poison ivy plants called urushiol (oo-roo-shee-ohl). .....the same substance that triggers an allergic reaction when people touch poison oak and poison sumac plants. Poison ivy, Eastern poison oak (, Western poison oak , and poison sumac are all members of the same family—”

Source.

I think the scratching “feels good” because it’s relieving the irritation of your nerves from whatever agent it is that’s causing the itch. However, I am neither a doctor nor a neurologist, nor do I play one on TV and this answer popped up in my head when I read your Q.

Scientists, back me up here please?

The nerves that send the message to the brain can’t tell the difference between something irritating your skin that needs to be wiped (scratched) off, such as an offending insect or leaf, from something that needs to be left alone.

Because of that lack of distinction, you have to try to constantly override the scratch impulse.

I don’t know. If scratching were so bad, it seems to me it would have been bred out of us. People would have all died off from all these infections and whatnot. I have to wonder if making yourself bleed might actually be a good thing. Perhaps scientists have not yet figured out why we scratch so much.

I think scratchign is a good thing. I’ve got the scars on my back to prove it. =)