I don't remember quite where I read it, but I have been thinking about a comment that connected the change in discipline (modifying the Friday penance) with the near catastrophic change in practice among the rank and file pew-warmers of ignoring the Church's teaching on family planning. From nearly 2 generations later, we can see how prophetic H.V. really was.
There are many young faithful Catholics who seem to think that there was some kind of Golden Age pre-Vatican II - an era where most of those who called themselves Catholic were well catechized through memorization of the Baltimore (or another) Catechism. I am old enough that I remember (vaguely) when Pope John XXIII died - and I remember some of the beginning changes brought about by V2 because they even affected my Anglican parish (like when they turned the altar around to face the people). My Catholic friends in the neighborhood and their older siblings were among the first generation to be affected by the V2 changes. My friend's parents had large families and joked about rhythm being Vatican roulette, but they were by and large obedient. So what happened to make my generation the most rebellious and dissident generation in the Church in ages?
I think that it happened because memorization, while extremely valuable as a foundation, is not the same as education. We memorize multiplication tables to give us the key to this mathematical operation - but we need education to understand when and how to multiply, and why. Multiplication without memorization is possible but difficult - but memorization without use is soon relegated to the dusty back corners of the brain.
So we had this large cohort of baby boomers whose parents were busy raising large families, and who trusted that Mary and John would get a reasonable religious education in the Parish school or CCD. Mary and John memorized their catechism, and learned that eating meat on Friday was a mortal sin, but not why!
For most mortal sins it is pretty easy to understand the reason the sin is so serious. Murder, adultery, theft - these all violate natural law and are also civil crimes, to boot. Missing Mass on Sunday is easily explained by a simple reference to the 10 commandments - keep holy the Sabbath. But eating meat on Friday? Where does that come into the picture? What made that a mortal sin?
The answer, of course, is that eating meat on Friday (or any day, for that matter) is not intrinsically sinful (no matter what PETA says). The sin was not in the eating, it was in the disobedience to a precept of the church, to a discipline that Holy Mother Church decided was in the interests of our spiritual health. But catechists and parents both seemed to take the approach of "Because I said so" and did not give the reasons behind the precept. (and there are many - starting with the idea that every Friday is a little Good Friday, and we would do well to be penitent and remember that). So when the Church decided that this precept was no longer accomplishing its goal (at least partly because fish has become more of a celebration and less of a penance!) it was not understood.
What I think happened is that a generation became confused and the Enemy took full advantage of that confusion. One day it was a mortal sin to eat meat on Friday - the next day it was not. So maybe the other commandments of the church about matters that are not civil crimes can also be relaxed?
And which widely practiced sins were at the center of that rebellion? The sexual sins. We did not have a well developed theology of sexuality and of the body. It was and is very difficult for the average person in our culture to see that contraception is inherently a sinful act, especially with all the propaganda about 'every child a wanted child' (which is translated 'every child a planned child' - though they are not the same). It is easier to see and be horrified by abortion, but it is difficult to explain why non-abortifacient methods of contraception are wrong - or on the other side, why periodic abstinence for the purpose of spacing or preventing pregnancy is licit in some circumstances.
If I had my way, every Catholic High school and CCD would have a study of Humanae Vitae and Evangelium Vitae.
About the time I realized the error of my ways and gave up contraception (about which I was not well catechized even or especially as a convert) I also decided to reinstate the Friday penance.