It was written by Solomon--as in, King of Israel Solomon, 700 wives Solomon, wisest man who ever lived Solomon, absurdly wealthy because everyone just gave him stuff for being smart Solomon. Previously, he had written the book of Proverbs, where he joyously declares that the universe is orderly, wisdom comes to those who seek it, and life is easy if you just think it through.
Then the whole 700 wives thing caught up with him and he became a pagan since he had married like hundreds of pagans, so God promised to remove almost all of Israel from his control.
Then he writes the book of Ecclesiastes, where he essentially pouts for several chapters about how life isn't fair, you'll never get anything you search for, wisdom is completely unattainable, so you might as well just eat drink and be merry instead of worrying about complicated stuff.
Sounds like Solomon just went through three stages of despair there. The rise, comeuppance, and fall of Solomon or something.
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