I feel that in the company of something true, a sadness could arise for all the time and energy spent — not only by one’s self, but by the multitudes — pursuing something false. Yet because it arises in the company of truth, it is sweetened by compassion, the seeing of which evokes another level of tears, and other qualities of sadness as well. In Zen, the term loneliness is used more often than sadness; they seem equivalent to some degree. You can look up sadness and Zen or loneliness and Zen and find different kinds of writings. Some of the writings refer to personal psychological sadness. However, I’m talking sadness that involves existence. I have not considered all the possible shades of sadness, so please offer your own in a comment.