https://www.newsweek.com/what-so-called-deaths-despair-exper...
A poll has limited value and people are famously bad at self-reported happiness. There have simply been too many cases of people saying they are happy with their lives and two years later they commit suicide, or kill their spouse. Some researchers have argued that self reported happiness is worthless.
Instead, look at actions. After the collapse of Communism and the breakup of the Soviet Union men over the age of 40 engaged in one of history's greatest alcoholic binges, such that life expectancy in Russia dropped for men during the 1990s. Actions speak loudly. Killing oneself with alcohol communicates how one really feels about the future.
We've seen something similar in the USA since the crash of 2008.
Until such time as we see real evidence of people acting in the way that we might expect happy people to behave, including planning for the future and taking care of one's health, don't believe a poll that says Americans are happy.
But still, in 1945, on VE day or VJ day, if you asked Americans "Are you happy?" obviously everyone would have said "Yes". This is well known. But what they were feeling was relief.
The same is true now. People feel relief, thinking the pandemic might be coming under control. Such relief is fickle. It could be different next month. It does not signal long term happiness.