Humans are not designed to be happy, or even content. Instead, we are designed primarily to survive and reproduce, like every other creature in the natural world. A state of contentment is discouraged by nature because it would lower our guard against possible threats to our survival.
The fact that evolution has prioritised the development of a big frontal lobe in our brain (which gives us excellent executive and analytical abilities) over a natural ability to be happy, tells us a lot about nature’s priorities. Different geographical locations and circuits in the brain are each associated with certain neurological and intellectual functions, but happiness, being a mere construct with no neurological basis, cannot be found in the brain tissue.
In fact, experts in this field argue that nature’s failure to weed out depression in the evolutionary process (despite the obvious disadvantages in terms of survival and reproduction) is due precisely to the fact that depression as an adaptation plays a useful role in times of adversity, by helping the depressed individual disengage from risky and hopeless situations in which he or she cannot win. Depressive ruminations can also have a problem solving function during difficult times.