Work is terrible, and the lives of many working people, even people with "decent" jobs in developed countries, are barely tolerable. It is currently socially unacceptable to mention this. Anyone who breaks that silence has done a good deed.
I've wondered about this a lot myself. Note along with figure 3 of the quoted article, according to a Gallup poll the average self reported life satisfaction in America is around 7/10. Presumably this average includes even including the sick/elderly/poor. I believe that my own self reported life satisfaction would be considerably lower than that if I were living the life of an average American.
I would guess that the difference is mostly accounted for by my own affective response to a given situation diverging heavily from the affective response that members of the general population would have in the same situation.
How confident are you that this reflects the experience of working people rather than how you would feel if you were in their position?
Somewhat confident. I work at a medical clinic. The number of people who come in with physical complaints relating to their job, psychological/stress complaints relating to their job, or complaints completely unrelated to their job but they talk to the doctor about how much they hate their job anyway because he's the only person who will listen - is pretty impressive.
From Mike Darwn's Chronopause, an essay titled "Would You Like Another Plate of This?", discussing people's attitudes to life:
Conclusion, graphs, and references in article. As usual, I recommend reading Chronopause.com as Darwin has many good articles; to quickly link a few: