I’ve learned that when I have a positive thought about someone, it’s generally worth communicating it. If I think a coworker saved me on some project, I tell them. If my partner is making me feel warm and fuzzy, I tell her. If someone impresses me, I say so. I don’t think people express these things nearly as often as they feel them, and I expect that many of the positive things that do end up communicated have another motive behind them. Simple earnest positive comments are very much worthwhile.