> What's the point of "being focused, exerting mental effort, being creative, having long-term goals or engaging in long-term endeavours, &c." if it's not beneficial to society in some way?
PG's statement [1] seemed to be about individuals, so the point is that doing x, y, z is beneficial to the individual.
I may have misunderstood, but the only other interpretation of his statement I can think of (besides "we were designed to work, and somone will feel bad if they don't") is "we were designed to work, and if we don't work enough, something will happen to our society or our environment, and then that something will cause most of us to feel bad", and that's a bit of a leap.
[1] i.e. "I think we were designed to work, ..., and we feel bad if we don't."
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> Also, PG is explicitly talking about people doing things that they enjoy at least to some extent.
I think it's somewhat beside the point, or at least it's not enough, to find some amount of enjoyment in one's work. Is it how you would choose to spend your time? Are there ways you can spend your time that would be better for you? Those are more important questions, I think.
And notably, PG is implying that we wouldn't choose to work enough, that the motivation we have is insufficient to get us to work as much as we'd need to not feel bad. After all, if we enjoyed working enough, we'd work as much as we needed, and there wouldn't any reason for PG to suggest that people need to be forced to work (by making an analogy with the claim that "the healthiest diet is the one our ancestors were forced to eat").
It's possible that for us to feel good, we need to be forced to do things, and that left to our own devices, as individuals, we'd never muster the cleverness to overcome our ill-serving desires or aversions. In fact, if we evolved in the constant presence of scarcities and abundances that our desires and aversions precisely counteract, it seems likely! [2] But why should we trust the business owners, who control their workers, or the wealthy or politically powerful, who want to maintain the status quo, to be the ones to force us? Why can't each of us pick who it is we want to help us by imposing structure in our lives?
[2] I actually don't believe this in the case of "not feeling bad", although I believe it for health. There is natural selection for behaviors promote healthiness, almost definitionally, but I can't think of any compelling reason for there to be evolutionary pressure for us not to suffer or not have contradicting desires that can't be simultaneously satisfied. In fact, designing intelligent life to be perpetually dissatisfied and wanting improvement seems like an excellent strategy for ensuring its perpetuation.