What does it mean to be sad?
It just occurred to me that this is the (pseudo?) existential question that is making me sad.
We need to first get up-to-date on the topic of sadness, therefore I would suggest this book:
"The Anatomy of Melancholy, What it is: With all the Kinds, Causes, Symptomes, Prognostickes, and Several Cures of it. In Three Maine Partitions with their several Sections, Members, and Subsections. Philosophically, Medicinally, Historically, Opened and Cut Up)" by Robert Burton (1621). With later updates, revisions and reprints.
(Usually just referred to as "The Anatomy of Melancholy", any good library will have it)
After you read Burton's book, you will certainly know what it means to be sad.
Then we would read a selection on existentialism. Maybe we could read Gyorgy Lukacs on Existentialism. Lukacs himself didn't think much of existentialism. You can find his essay on the internet. I don't think existentialism will help us much with our sadness, it may even make it worse!! However, there are some people who can truly commit to something so much that they rarely get sad.
Then again, we may just be expecting too much from life. Everyone smiles in the advertisements on TV and on the net so we think everyone is happy all the time. But remember, the people smiling are actors, and a director is yelling at them to smile for the camera!
This pretty much leaves us with the Buddha who said that this world is suffering, it is all about suffering, and then he laid out the eightfold path as the way to deal with the problem. But enlightenment requires us to let go of attachments. It is very, very hard for all people, much less for Westerners, to let go of attachments. Especially the attachment we have to our own ego. Nevertheless, the Buddha did some very profound thinking on this topic, and it is probably worth studying him.
I don't mean to slight the other religions or belief systems. I mention Buddhism because it puts the issue front and center it seems to me.
NB: The Wikipedia for The Anatomy of Melancholy quotes a literary critic as saying that the Anatomy "survives among the cognoscenti." And that is certainly true. Those in the know will have read this book.
From a basic psychoanalytic point of view, a la Melanie Klein, sadness is a regression to the depressive position that follows from the deflation of the infantile delusion of omnipotence -- the idea that things happen in the world just because we want them to.
The idea is that we come into the world with no notion of desire or satisfaction. We reflexively behave as we naturally behave and we are occasionally satisfied. With no relation to others, we assume that good things happen just because we wan them hard enough. So whatever we want, we will be able to get it whenever we need it enough. We are omnipotent.
At some point, we realize that there are independent causes of the things we want. We are crushed, because this presents the idea that there are things we need that we may simply never get. This is the point where we discover that our actions, primarily crying, do actually affect the actions of those external causes. This is our first sense of the basic notion of responsibility. We now have a reason to cry when we want to, and not to waste our resources otherwise.
For Klein, sadness and grief are regressions to the state in which we have first learned when we want to cry and when we don't. Particularly the realization that further action on our part may not be effective. This would come to us in the state of having cried all we can manage and still not being satisfied or comforted enough to go to sleep. The idea that they evoke this very potent early memory explains both the feeling of exhaustion that accompanies them, and the fact their external emotional expression is to cry, but not energetically.
(For brevity and continuity, I have skipped over the 'paranoid/schizoid' position here, during which we are omnipotent and contain the satisfying other, but face some kind of equally omnipotent thwarting other and there is no logic to life. But I need to admit her actual logic is not this compact.)
Here's a fragment from an interview with Agamben which might throw some light on your question:
interviewer: Is this vision of becoming human, in your works, not rather pessimistic?
Agamben: I am very happy that you asked me that question, since I find that people often call me a pessimist. First of all, at a personal level, that is not at all the case. Secondly, the concepts pessimism and optimism have nothing to so with thought.
Debord often cited a letter of Marx saying that 'the hopeless conditions of the society within which I live fill me with hope'. Any radical thought always adopts the most extreme position of desperation. Simone Weil said 'I do not like those people who warm their hearts with empty hopes'. Thought for me, is just that: the courage of hopelessness, and is not that the height of optimism?
There again, it might raise more questions ...!