system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we have itches and why do we need to scratch them?
gpt
Itches are a reaction from your nerves. Some things can stimulate these nerves (histamines, for example) in a specific way. Scratching is, put simply, temporarily deadening those nerves. Some stronger histamines, allergens, poisons (stings, bites) have a more pronounced effect, hence more scratching to "deaden" the nerve it's affecting.

tl;dr, itchiness is a specific reaction from your nerves, and scratching sort of deadens it.