OP, that is a very big question, and it does not have a simple answer.
Europe went through two World Wars in the 20th century. These visited unprecedented brutality and some grave privation on the civilian population during the early part of the century. This had the effect of weakening the faith in Europe. The spread of certain humanist philosophies also weakened the faith.
In the United States, young people growing up in the 1930s and early 1940s learned to live in frugal circumstances, in a society that was in frugal circumstances. On this group of young people fell the demands of World War II.
During World War II, there was a great deal of work that needed to be done for the war effort that had traditionally been considered “men’s work”, but that had no male laborers to do it. This work fell on a generation of women that had grown up during the Depression, a time when you coped with whatever fate fell upon you. Consequently, women stepped up and worked outside the home in traditionally male occupations: welding, driving truck, and so on. Where the job could not be done with the physical strength of a female, ways had to be found to do the job without it. Do I need to tell you that once women realized they were fully capable of doing the things they had been told they could not do, that this genie was not going back into the bottle? Similarly, blacks and Japanese men had served their country well, had shown that they could fight and die for their country as well as men of European ancestry. That genie was not going back into the bottle, either. The men who had gone off to war would find everything changed when they got back.
As for the men, a great many of them, young married men with young children, did not come back. Their widows had to go to work as single mothers to raise their children, sometimes without re-marriage (who would they all have married?) The men who came back were deeply changed, going into war as a teen or twentysomething and seeing their closest buddies die in horrific ways, seeing cities in ruins, and found their world totally changed when they got back. Yet the returning men were regarded as heroes and the attitude was, on the surface, one of unmitigated triumph. This, after all, was a generation and a nation capable of great things, a world power! Very few of the men could talk to anyone who had not been in those battles about what they had been through. It hardly needs to be said that the parents of that age came out of the war with a maturity unusual for their age, but also with inner wounds that they never talked about. This had effect on their relationships, including their relationships with their children.
Their children grew up in a totally different atmosphere. They knew little want and many opportunities. They knew few of the privations and demands their parents had known. Because of the GI bill, college had become a middle-class expectation. For this generation, the world was their oyster. It was the first generation with a middle-class “intelligentsia”. They went home from college full of new ideas, but not ideas welcomed by their parents. In their young adulthood, they did not see optimistic movie house news reels of a war being won, with the gore censored out. They saw the full gore of war, in their living rooms, gore that included women and children, and from a war whose aims could not be easily articulated, in which it was far more difficult to show the freedom that the blood of the young was buying. Viet Nam was also a war in which the young were aware the upper classes were not fighting. Harvard and Yale and other universities sent many young men off to fight WWI and WWII. The same wasn’t true of the Viet Nam conflict.
All of these things drove a wedge between the old and the young that was unprecedented. Many people born after the war lived by the mantra “Never trust anyone over 30.” They accepted nothing of what their parents tried to tell them at face value, but questioned everything. They thought that for the good of the world they ought to re-invent everything, including ethics and religion. They thought they knew better. Suffice it to say that although they did usher in many good things, such as civil rights, they also threw out the baby with the bathwater, discarding the true wisdom their parents knew, too.
As others have pointed out, their lives were also changed in unprecedented ways because of modern medicine. The birth control pill, yes, but not just the pill. Vaccinations, antibiotics, and other advances made huge changes in childhood mortality rates. Death was fought tooth and nail, with the expectation that medicine might one day defeat it.
The list can go on and on. Suffice it to say that the 20th century was a very difficult one, a century of unprecedented social upheaval. It had its advances, but there were also profound set-backs that are only being appreciated now, by the children of the baby boomers. People will blame this on many things, but I think it is clear that there wasn’t an easy way through that century.
IMHO, it isn’t our task now to criticize the 20th century, but to revive those worthy institutions and wisdoms that it tried to bury. We’ll make our own mistakes and have our own failures in this century. If we are humble when we consider the mistakes that came before us, perhaps we will have fewer of them.