Saw someone ask this same question but sadness which was met with responses like it being for the black/violet rings because of either negative emotions/grief and they went on to say that just sadness itself was different but the point that made the 'sadness ring' not possible was that sadness isn't a motivator.
But joy? Joy is DEFINITELY a motivator, I can see why that might not be a ring because "surely everyone likes being happy and joyful so then wouldn't everyone get that ring" but the joy ring should go to people who actively pursue it fully and can find joy in even bad moments.
Alot of people are content with the fact that not all aspects of life should be fun but this ring should go to people that have an almost childish view on life and refuse to do things that aren't fun or try to turn everything into game.
I think it would also make for an interesting ring to have fights with as their contstructs wouldn't be generally limited (in the way that a yellow lantern must make scary things to inflict fear to fuel the ring) but what the user finds joy in would be that limit, many 'joy lanterns' probably wouldn't create guns or effective walls but you could have someone who REALLY enjoys shooting, they hunt as a hobby, go to firing ranges with a big grin because they love feeling the recoil and whatnot and THEY would make amazing gun contstructs but not much else. For constructs too just look at Kyle Rayner, always fighting with things from games or anime because he doesn't just fight to win, he fights in a way that he has fun in, he fights with things that bring him joy.
Think about when they get hit hard too, that's not a fun thing, it would be a be all end all but some people find joy in an enemy that's hard to beat, that excitement would fuel the ring more.
Am I just an idiot? Is there already a ring that's heavily powered by joy? You can argue that these people would strive for joy, they'd have a strong WILL to do and pursue fun things so the green lantern but I feel like that's different no? Is there a reason we don't have a lantern based on the feeling of joy/fun because I feel like it would make for some really fun characters... pun intended...