system
Given the following post, respond as if you're commenting on Reddit:
human
[Green Lantern] How was it decided that the rings would be assigned the specific emotions?

Why couldn't there be a ring for happy or sadness?
gpt
The beings that represent each emotion were the first to harness that emotions power. For example Ion was the first fish with the willpower to walk on land. Parallex was a wasp, the first thing to be so scary that it harnessed power from it. 

All of the emotions harness a power, that's what the rings are tapping into. Sadness was never harnessed a power. Those who turned their sadness into rage on the other hand.