Today our a lot of society and culture is based off subjectivism. I can't tell you how many times people have discarded what I have said simply because "it's not true for them". Truth is truth. Good is good. Lewis warns us that we can not and should not try to create our own values. If we all have our own little values and beliefs, then what is true? What is the right one? I feel secular people may say: "Whatever makes you happy." But that does make an absolute truth, and it never will. Happiness can change. Truth cannot.
However it is not just a secular problem. Christians too might start using subjectivism in their own ways. We have to be careful to remember the fall of mankind.
"To say a that thing is good is merely to express our feeling about it; and our feeling about it is the feeling we have been socially conditioned to have."
This is warning from Lewis. Good will not be judged on the basis on what good actually is, it will be based on what "we have been socially conditioned to have." There are many things that are bad about this. 1) it gives way to much control to others and our environment. Because one sees something on TV and the producers have made a bad act looked good. This has happened with premarital sex. All of TV and movies we see people having premarital sex. And the producers make it seem good. And the result is most people thinking that it is okay to have premarital sex as long as you love person. It is a good thing. Which in reality is only a good thing when it is in the context of marriage. 2) "Our feeling about [good]" It's a feeling. Feelings change all the time. To judge whether or not something is good based on feelings is not absolute. One needs to judge on facts. If one judges on feelings the judgments can change. It's just a no good circle of nothing this subjectivism.