I can think of a few reasons for this besides the reason that life has genuinely become worse:<p>1. Profit-seeking news orgs: it's no secret that negative and controversial news sells significantly more. With all the destruction of locally run papers in favor of the national consolidation of news by a lot of private equity investors, there are increased expectations to make a profit and therefore increase the amount of negative-sentiment news over the recent years.<p>2. 24-7 news cycle: people are also much more aware of all the bad news around them with smartphones and social networks. Readers' sentiment will just bleed into the papers over time. Ignorance is bliss.<p>3. Sampling bias: which kind of papers did they measure sentiment for back then versus now? There could be a divergence in the sources they use and different sources could have different sentiment tendencies. (I don't have access to the actual paper)<p>4. Rising expectations: humans are many orders of magnitude more powerful than our ancestors. We live like gods compared to them. Yet there are so many people who still aren't happy. Why? It's because our expectations also rise endlessly. Things may be better than before, but maybe <i>it's not better relative to our expectations</i>.<p>Point is, this isn't necessarily indicative of life becoming worse. There could be other plausible explanations.