This is my theory. Humans are the most successful primates because their superior ability to adapt. Adapting though also means that whatever state is reached it becomes normalcy and its virtues no longer appreciated (e.g. the benefits of living in our modern times are taken for granted). Therefore, for it there be happiness there must be pain. Work provides the necessary pain from which happiness emerges. Without it, I suspect there would be only existential crisis, depression, and war. Ask yourself: do you think the average insta-citizen would settle into a state of content if left with little or no work to do, free resources (i.e. universal income) and to their own devices? I posit not.
