Well, I was going to say vitamin D because a lack of vitamins can cause depression and getting them can cure some ailments/prevent them. You need to not be in that depression to express happiness. It’s something worth taking care of, or occasionally extremely important, but rarely an actual issue. You get it from the sun and I guess the sun reflected from the moon could supply some Vitamin D.
Unfortunately, my curiousity to see if my first guess was correct, after a very short time contemplating, lead me to Google the riddle and the common answer is the letter S. Shame, just before I Googled the riddle I noticed that is a possible answer, but didn’t really care because I’m almost done a digital painting I’m working on.
So yeah… the letter S, you curvy mother fucker.
Wait, why am I being riddled?
I wish it was vitamin D. That’s totally mostly correct. Clever of this riddle to attempt to throw off the reader by associating everything emotions though.