The next principle is: give honest and sincere appreciation.

The point is, that if you make people feel legitimately important, then you'll be able to influence them.

He claims, that all people have a desire to be great.
"Tell me how you get your feelings of importance, I'll tell you who you are. That determines your character."

Here's another quote from the book:
"People sometimes become invalids in order to win sympathy and attention and get a feeling of importance. Some authorities declare that people may actually go insane in order to find, in the dreamland of insanity, the feeling of importance that has been denied to them in the harsh world of reality."

So, I guess, if you appreciate someone, but don't flatter them, you could save their mental health?