Adult sadness is like a medicine locked in a safe. When it can be shown to others, it is either an antidote and a warm sun for the future! Either a dose of poison, an end!
The reason why sadness is sad is that there is a negative contrast, because only you can feel it, and because the essence of sadness is that you can bear it alone!
Concept: extreme happiness brings about sadness, then in turn, extreme sadness should bring about happiness