Treat a person as he is, and he will remain as he is. Treat a person as if he were where he could be and should be, and he will become what he could be and should be.
There is nothing to compare with the instantaneous feedback a singer gets from the people sitting in front of him. That is where it all comes together - all the rehearsing and working to get everything just exactly right.
I can't change the direction of the wind, but I can adjust my sails to always reach my destination.