>I see absolutely no reason to believe that the natural state of humanity is to be happy (or, more specifically, content)

What about the fact that stress is tremendously destructive to our health? It is directly a threat to the survival of both individuals and the societies they live in. If being in an anxious state for extensive periods of time were the most advantageous to survival, then evolution seems not to have noticed it. The structure and functioning of our endocrine system heavily suggests that humans were built for occasional short bursts of high stress, not extended periods of constant high stress (as we have designed society to require of most people).