I feel very strongly about giving praise and encouragement having not recieved much from at least 5 of my own teachers. It is critical that students see that what they are doing is worth something if you constantly show disappointment or disgust at their playing they will end up depressed as I nearly did.
Yes, but saying something is wrong isn't showing disgust or disappointment. Saying something is right isn't praising it or encouraging it.
That's the objectivity you need to get. In an ideal world. Your piano teacher would tell you what your playing is like, and how to improve it, and because they did that in an objective way - without emotional baggage or "acting disappointed" then you'd eventually trust them to be objective with you and take their comments for what they are - honest comments about your piano playing and a way of improving it.
You'd learn from their honest comments when your playing was good and when it wasn't and develop your own judgement. e.g Folk who are convinced they are bad on the piano didn't need people saying their playing was great, they needed people saying what they playing was really like. "Trust" is the thing, but they may have trusted someone who danced around the truth worrying about feelings and the ratio of praise to encouragement they should give rather than just being honest.
You wouldn't be misled into thinking that bad playing was good because they praised it, nor that bad playing was something to feel negative or bad about.
So when they say "You're making mistakes in that section, slow it down" or whatever, you take that for what it is, it's not saying "Your playing stinks, you are fat and ugly and no one likes you, I'm disappointed"
But in the real world, people, including the teacher, will take their emotional baggage to the lesson. Our poster here gets frustrated by hearing bad playing and thus might say something that isn't objective one day. Similary, his pupil may react to that, or to purely objective criticism in an emotive way because of their past experiences.
I don't think the solution to that is to err in the other direction - to start showering praise on a performance in an attempt to balance some perceived "bad piano teacher history" - if you are going to be disgusted or disappointed or praise them, at least make it an honest reaction, not something you're acting out because it sounds like a good idea to throw your arms in the air and whoop every 3.45 lessons to up moral according to some forum.
Because that, to me, just says that you aren't being objective. If you aren't objective when you say it's right, you probably aren't when you say it's not. That's the message you give. Thus the way they take negative criticism from you may suffer.
Analogy : Imagine you're a dietician, you have a kid and they are overweight to the point of making themselves ill. Do you say (a) "Hey, you're a porky git, who ate all the pies? Stop eating you greedy sod you disgust me" / (b) "You look beautiful, bonny, it's puppy fat. Everyone is good at something, fitting through a door isn't everything!" or (c) "You weigh 354 lbs and are 3'5" you need to lose weight because at your height you should weigh between x and y and it's affecting your health. Here's a plan and a bunch of useful. honest and objective stuff to help"?
I think the kid will find lots of people to say (a) and (b), they don't need a piano teacher for that, worse, they might believe the piano teacher that says (a) or (b) more because they are supposed to know.