(To start off, this is not meant to discredit the validity of mental illness diagnosis or to deny the suffering of those afflicted. To say that there is an evolutionary reason for something is different than saying it isn't a problem or shouldn't be fixed.)

Society basically says that you are supposed to be happy all of the time, or most of the time (unless you have a reason not to be). Contemporary psychology says that depression (and other mental disorders) are illnesses.

Personally, I have bipolar disorder (mostly depression). When I talk to a lot of (normal) people about my feelings, they seem to share similar sentiments (although probably not as severe) that life is hard, you have to suffer a lot, you have to work for happiness, etc.

When we look around, mental illness seems to exist in every society. An alarmingly increasing number of people are being diagnosed with depression (and other mental illnesses) and prescribed antidepressants. It's not even an insignificant amount of the population.

So this led to me to question if humans were "meant" to be happy all the time, evolutionarily speaking. Were our human ancestors happy all the time, or is this a modern construct? Is a state of mild depression our natural state? And could this be evolutionarily advantageous?