When we worship God in the way He demands, we use certain words and phrases. These are not for God's benefit, because He doesn't need our worship. They are for our own understanding and benefit.
The changing prayers of the Mass have been part of the Catholic Church's vocabulary for nearly 2,000 years. Many of them were written more than 1,400 years ago and reflect the teachings of the Apostles.
But in the 20th century, many of these ancient prayers that were present in the Mass and on the lips of our great-great-great grandparents have been deleted. The ones that made it into the Mass of Paul VI were revised to "highlight new values and new perspectives."
The Second Vatican Council supposedly sought these revisions. Its first document, Sacrosanctum Concilium, made only a few vague demands to reform the Liturgy, namely:
The engineers of the Novus Ordo Mass took the opportunity to de-emphasize the evil and dangers of personal sin while emphasizing other aspects of the Faith. They did this purposefully and with certain intentions. Most of the substitutions were taken from ancient Catholic sources as well, but they were jammed in by the new liturgical engineers.
As a result, the original poetic flow of the prayers was broken and their theological sense changed to say something the original prayer did not intend to say.
None of these engineered prayers are wrong or immoral, nor do they necessarily contain any theological errors. Also, nobody is sinning by giving their assent to the prayers when they answer "Amen."
But the resulting change of emphasis during the past 50 years has downplayed the necessity of repentance. The results are ever more obvious in the Church and society.
To learn more about who did this and why, watch Reclaiming Tradition: Changing the Vocabulary.