Whether you are talking to a friend, dealing with family, or buying something in a store, feedback is important. If someone has done something nice for you, feedback is a positive reinforcement. If your expectations have been disappointed, sharing your feelings can help the other person do better in the future. 
Even as a psychologist, it is important to have feedback. I tell all of my new clients that if they are comfortable with me, we will set goals for therapy together. But if they are not comfortable, they should tell me and I will help them find someone whom they think is a better fit.
Recently I was delighted to get wonderful feedback from a client I had not seen in years. I sent congratulations regarding a work anniversary on a public forum and got the most amazing note in response. The client said (and I quote this with permission,) “I really have no idea where I would be without you and can’t thank you for all of the help and support you gave through a very pivotal time in my life…every milestone I have had I always think back to our sessions…I wouldn’t be where I am without you and just wanted to let you know the impact you had on my life. “
It is rare to hear from clients after they end therapy. And it is even rarer to get such touching feedback. It reminds me that I do something important and that I have the ability to make a real difference in people’s lives.