system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is an itch? Why are some itches so intense that you need to scratch them immediately?
gpt
Itching is perhaps best thought of as complementary to pain. They involve similar neurons and chemicals. Importantly, though, while pain makes you withdraw from whatever's causing the pain, itching makes you want to scratch the area. (Also significant: Pain inhibits itching.) The strong desire to scratch is possibly evolutionary--parasites cause itching, and scratching may help remove them.