I think the fact that happiness is (mostly) zero sum comes from the fact that in early humanity's evolutionary environment, the ecosystem had a mostly constant

I think the fact that happiness is (mostly) zero sum comes from the fact that in early humanity's evolutionary environment, the ecosystem had a mostly constant (meaning over human reproductive cycle timelines) carrying capacity for humans. Innovative advances in technology or techniques or knowledge that would allow for more humans to thrive with better qualities of life were so rare that evolution did not have much opportunity to select for them. In that kind of environment, one's genes can only propagate at the expense of others'. So we're now all the decedents of the early humans who preferred being the top dog of squalor as opposed to an equal in a prosperous society.