Someone wise said something like: "the primary feeling in any interaction should be love", and, "never let a problem to be solved become more important than a person to be loved". It takes practice, and probably a supporting belief system. They didn't mean romantic love, but a sense of really caring about the person and their well-being, as more important than any putting them down or $ or whatever.
I think I said that better at my web site (in profile), under something like "Other / Life Lessons learned" / maturity models / social / ...."
Edit: another way to look at it is to be sure to say what you are NOT saying: "First of all, thanks for your efforts. One thing that happened is X did not actually help with Y, but it might in the future if it were more Z. (smile) I am definitely not saying you are are a bad person or something, or should be fired [if you can say so honestly]. We all have room to improve, I know I do, and I hope you will tell me so when you see ways I can improve. And I appreciate your efforts, and you have really good potential. Let's go forward together on this project from here." (Everyone does have good potential, in some context, I believe, and maybe it is almost always good to say "I think we can go forward on ___ with ___. I can be wrong, and thanks for listening!! That is kind of you. I will try to do the same." [then do so, asking questions, you can always say "interesting, I should probably give that some thought"... in a pinch.)
There are many good ideas out there. Asking questions and listening and observing, among them.
(some edits.., including correcting navigation notes about my web site)