A billion dollars would give you all the freedom you need to be happy -- but going from that to "a billionaire must be unusually happy", or even "most billionaires are unusually happy", seems to depend on a lot more self-awareness and agency than I think most people of any income actually have.

One thing I muse over sometimes in the context of billionaires is that, by and large, we should expect them to be strange and often unhappy people - simply because anyone more normal and well-adjusted would have stopped at, say, $10 million and $10 million typically doesn't accidentally turn into billions. Continuing past the point where all one's real needs are met indicates a bizarrely low estimate of the utility of switching to consumption and away from earning additional money (or perhaps the inability to stop working).