From Scott Adams, the creator of Dilbert, in How to Fail at Almost Everything and Still Win Big:
Children are accustomed to a continual stream of criticisms and praise, but adults can go weeks without a compliment while enduring criticism both at work and at home. Adults are starved for a kind word.
When you understand the power of honest praise (as opposed to bullshitting, flattery, and sucking up), you realize that withholding it borders on immoral. If you see something that impresses you, a decent respect to humanity insists you voice your praise.