In the beginning, people were mostly unhappy, but not too unhappy about being unhappy. They hunted, they gathered, and when unpleasant things (such as having a leg bitten off by a lion) happened, they shrugged their shoulders and said, “what are you gonna to do, huh?” And they spent as much time as they could being idle, because that seemed to help them not be unhappy for a while. This worked particularly well when there were temporarily no lions around trying to eat them.
Then history began to happen.
The people who first noticed there was history going on — they were called poets — also discovered that it ruined idleness for them (this effect would later be named “the frame problem”). This made them very angry, so they decided to tell everybody about history. If they couldn’t have any fun, why should anybody else? They also decided to write some of it down, just in case their children, and their children’s children, tried to forget the discovery. Future generations, they figured, had a right to remain innocent of unnecessary and burdensome knowledge of events past. Perhaps some pleasure could be found in denying them this right.
There was nothing much they could do about the fact that their ancestors in their graves, unlike their descendants, were beyond the reach of their words. But in a stroke of genius, they realized they could make their descendants more miserable by pretending that their pre-historic ancestors had actually been continuously happy, instead of just free of unhappiness about unhappiness.
That made it look like it was all going downhill, which made the poets happy about being unhappy about being unhappy. Because at least those who came after would feel like they were even worse off.