People crave appreciation more than they admit. When it is given honestly and specifically, it satisfies a deep need and makes the recipient far more receptive to influence.
Carnegie distinguishes sincere appreciation from flattery. Flattery is obvious and self-serving; genuine appreciation notices real effort or quality and names it.
The technique works because it makes the other person feel important and valued. Once that feeling is established, cooperation and goodwill follow naturally.
This principle appears early in the book and is the foundation for almost every later method of winning people over.