In the WWs you had a society perhaps more structured and united for a common national and patriotic cause.
In 1968 (since it's mentioned) and perhaps in recent years you have a society that arguably is less structured and people having issues with society itself and the state of the country.
It also seems to me that it was during the 70s that the US started to feel that things and the country were going down.
I suppose there is a parallel in the UK, for instance. Things were hard during WWII but people came together and had a sense of purpose and of fighting for their homes.
But then factories close, you lose your job, inflation picks up, society changes... now you may perhaps really feel depressed.