system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does constant pain never go away, while constant pleasure eventually fades?
gpt
We get used to pleasure through a process called affective adaptation or hedonic treadmill. But this same process also applies to sadness. We need our emotions to make decisions, which means our brains apply the old adage that it's "No use crying over spilled milk" and forget about the past and how happy or sad we were; worry instead about the near future.

Pain isn't really the opposite of pleasure, so different mechanisms are at work. If something hurts you, you want to become more sensitive to it so you "learn your lesson" and can better avoid it. So the brain does what is called central sensitization to pain, so the pain can actually get worse if it persists for a long time. This is great for avoiding painful plants in the jungle but terrible for permanent injuries that cause pain.