If we aren't designed to be happy, then the dopamine-reward hypothesis is a complete evolutionary anomaly. The same goes with oxytocin production in the brain; there's no natural inclination to either happiness or unhappiness. If we were meant to be unhappy, these two chemicals, and their following effects and reactions wouldn't exist.
Depression, to me, is a product of chemical imbalances in the brain and environmental stressors (poor home life, social life, financial worries etc etc). Whether the chemical imbalance brings about the environmental stressors or vice versa is purely down to the individual case. What can be said is that due to humans purely unique idiosyncrasies and behaviour, we may never find a general unifying theory as to what causes depression, or any other mental illness; because we all process emotions differently, we all react to emotions differently, and no brain works the same as another.
Also just to add, that article seems to suggest that we should just become stoic mages; devoid of any sort of positive expectation from our lives, not good advice.