This is a bit of a provocative answer. It also holds such profound implications. It's true both on the surface, as a statement, and also as an indicator to the type of relationship that must be present for therapy to foster significant change.
As therapists, we rely on our clients for the content of therapy. We are not mind readers or gurus who tell our clients what is happening. Instead, we are trained to listen, interpret and share what we are hearing and where our mind goes with it. So if a client does not bring in the uncomfortable parts of what is bringing them to therapy, we may realize that something like that is happening but we will not then know what those uncomfortable parts are. We need our clients to take those risks.
Secondly, the therapy between the client and therapist inevitably includes interpersonal patterns that are part of a clients life. It is essential that in therapy we address those patterns, develop insight into them and rework them. To do this meaningfully, the therapy relationship itself must be discussed. At times, this might even include uncomfortable feelings like anger. If a client is unwilling to speak up about feeling this way or the therapist is defensive, the amount of meaningful change that can take place is impaired.