Criticism is futile because it puts the other person on the defensive and wounds their pride. No one likes to be told they are wrong, and the moment criticism begins, useful conversation usually ends.
Carnegie observed this pattern across thousands of students and business situations. The person being criticized rarely changes their mind; instead they justify their position and resent the critic. The relationship deteriorates even if the criticism was factually correct.
The alternative is to understand the other person's point of view first and to avoid direct confrontation. When change is needed, it is far more effective to let the other person save face and discover the better way themselves.
This principle underpins the entire book. Almost every later technique exists because direct criticism does not work.