My second guess would be politics. I have met few people in the last few years that do not seem unhappy as a direct result of our political battles. Families actually breaking up over it, etc.
Now I will go read the article ;-)
There's been a massive increase in high risk behaviors, an increase in road rage, and a spike in traffic fatalities since COVID.
If COVID brain damage affects motor vehicle operation, it wouldn't be so far fetched to say it negatively effects happiness and overall wellbeing. Covid causes a loss of grey matter affecting impulse control and emotional regulation.
If millions of people have brain damage affecting impulse control and we are all collectively quick to anger now, which will manifest as collective frustration and unhappiness.
Not unlike the theory of Lead poisoning causing crime in the 70s and 80s. Our generation may be suffering a similar fate as a result of COVID.
It seems this statement is not fully supported by the data. While there have been mixed studies linking COVID with impacts on grey matter, we can't conclude that COVID infections have impacted grey matter to the degree that it has "affected impulse control and emotional regulation".
It seems more likely that collective stress increased since 2020 due to economic gyrations that have inordinately benefitted the wealthy while the poor and middle class suffer. Governments and society have been quick to dismiss those financial and economic stresses, including efforts to minimize the true realities and impacts of high inflation.
Telling people "you're not financially stressed, you're just brain damaged!" seems like further perpetuation of that gaslighting happening to people in society who are legitimately suffering due to structural disadvantages in the economy.
Not to mention the COVID-era destruction of social connections, third spaces, and lockdowns that promoted increased smartphone reliance/addiction, and increased alcohol consumption. (Schools closed, liquor stores open)