system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does scratching an itch temporarily relieve it only to trigger more itchiness afterwards?
gpt
When you scratch at an itch, your nerves transmit pain signals to the brain. This blocks this itching sensation temporarily. It's like you are distracting your brain and causing it to pay attention to something else.

The brain responds to pain signals by releasing the neurotransmitter serotonin. A study done at Washington University School of Medicine in St. Louis shows that serotonin may cause the itching to get worse. The researchers had two strains of mice. One strain was incapable of producing serotonin. Both strains were injected with a chemical which is known to cause itches. The mice which couldn't make serotonin didn't scratch.

So, basically: When you scratch, it hurts. Your brain releases serotonin in response, which causes the itch to get worse.