I have a theory about why this is so, and here it is: the reason we, as a species, are unhappy is that we're not entirely sure that we exist.
There is some classic philosophy behind this, which can be found in both the occident and orient. The argument is something like this: For something to truly, concretely exist, as a phenomenon it would be concrete, definite, tangible. Moreover, as some philosophers have tried to show through logic, something that truly "exists" would not materialize or dissolve, but would have stable, non-ending existence.
Since none of these qualities can possibly be applied to the concept we call "self," we exist in a kind of nervous worry about who and what we are....or even IF we are.
Consider the evidence. In almost every circumstance, who we are changes moment to moment. One moment I'm a husband, the next a father, now a friend, later a son to an aging father. A supervisor, an underling; an intellectual, a screaming sports fan. Even in the absence of other people and changing circumstances, the self I identify in my thoughts changes every few seconds.
Our emotional view of the world, which may define us more than almost anything else, is the most shifting experience of all. Blissfully content one moment, irritated the next. Happy as a clam today; on the wrong side of the bed tomorrow. Full of wisdom and common sense....30 seconds later forgetting where I put my eyeglasses.
25 years old one minute, a second later, I'm 53 years and counting.
The reality truly is that there is a new reality with every passing moment, and that nothing whatsoever is real in the sense of being concrete and stable. In the very moment that some phenomenon occurs, it is already vanishing. No wonder we're all a bit anguished about our place in the cosmos.
That's the real nature of things: flux, change, impermanence. Unhappiness, it seems to me, arises because we don't like the real nature of things, and we're constantly fighting against it. It's not much more complicated than that.