Thing is, in primitive, savannah environment all problems were simple enough that they could be solved by just few hours/days of thinking and planning. Nowadays we live in civilized society which poses much bigger challenges, which cannot be easily fixed. That's why depression can extend to weeks/months, and after that time it becomes chronic condition, even when the actual problems that caused it has already gone away.
But I would also argue, I that even in primitiv societies, things could get very complex - because their daily life involved also lots of deamons, spirits, ect. you had a bad dream, but that might mean that an enemy sorcerer tried to kill you and now you needed to figure out who and what to do against it ...