When would you say this to someone?
"Thanks for doing such a great job on this project. I'm happy you're taking on more and more responsibility."You probably think you'd say this when someone has impressed you with their performance. But what if the person's performance is merely borderline acceptable, and you give some constructive criticism, and yet you still say it? Then you've performed a Jedi mind trick.
To the human subconscious, a statement like that is a powerful command. And I bet you know exactly what I'm talking about, now that I've brought it up: When you tell people they're making a positive impression on you, they automatically turn up the behavior that caused it.
If you want to get more out of the people around you, you need to start the virtuous cycle by noticing when their effect on you is even the slightest bit more positive than usual, and acting pleased.
And if there's someone who never pleases you, maybe part of the reason is that your consistently cold reactions to that person have flattened the reinforcement gradient that would normally amplify their positive behaviors over time.
Be appreciative whenever anyone behaves in what you judge to be the top 20% of their range. And if you want your reaction to cause an even bigger shift in their motivational dynamic, then kick it up a notch: be impressed.