The relationship between a mental health professional and a patient is called a therapeutic relationship. It is the means by which a therapist and a patient hope to engage and effect beneficial change.Therapy is essentially an unbalanced relationship: the patient opens up and the therapist usually does not. This is necessary so that the conversation focuses exclusively on the patient's problems and not the therapist's life.
However, how can trust develop in such a one-sided relationship? Because the therapist doesn't reveal as much about him, you're expected to see him as a safe, caring listener dedicated to helping you discover your problems, not his.
Normal social friendship, on the other hand, is bilateral. In most relationships, we open up gradually as the other person opens up too. As your friend, the person knows a lot about you and you know a lot about him. We usually share experiences beyond just sitting in the room, talking. Therapy can certainly be a friendly relationship, depending on the personalities involved and the therapist's theoretical orientation, but it should not cross professional boundaries.