Here is an excerpt of a note I post to my students in the first couple weeks of class:
One of the general characteristics of the instructor student interaction is constructive criticism. It may seem, at times, as though your instructors are only giving you feedback on what needs correction instead of what is going very well in your work. This happens naturally when an instructor is trying to help you see something new, or help you develop a concept. If you feel like I (or another faculty member) am only giving you feedback that is critical please feel free to ask me what you are doing well. I try to remember to give positive reinforcement but sometimes I am so focused on pointing out concepts that I forget to encourage. The good work that this class is doing does merit encouragement! Keep up the good work and keep working hard.