Of course, a patient should tell the therapist anything and everything that is relevant to the problems at hand and to the therapy. But it is never that easy. It may be easy for me to tell the mechanic everything about the problem I have with my car or the dentist all the relevant information about my problem tooth -- I don't feel personally at stake in those transactions because everyone develops car trouble or dental problems at least once in their lives. But in therapy, things cut much closer to the bone, especially when the therapy is psychodynamic or depth oriented. Because then we are not talking just about observable behaviors or discrete problems, but rather about innermost feelings and thoughts.
The basic instruction of psychoanalysis, "Say whatever comes to mind" is both extremely simple and fiendishly difficult. It means letting go of the rules we all have about what is and isn't all right to say, what things we can and cannot admit to. Just try it some time and see how quickly the inner censor starts editing what you feel you can say.
"You see, the Self is such a disagreeable thing in a way, so realistic, because it is what you really are, not what you want to be or imagine you ought to be; and that reality is so poor, sometimes dangerous, and even disgusting, that you quite naturally make every effort not to be yourself." C.G. Jung
It takes time for most people to build the depth of trust needed to feel secure enough to talk about anything and everything. It takes the experience of trying first this, then that and discovering that what you feared would happen didn't, that the therapist can and does still care for you despite whatever dark thing you have revealed. Each successful experience lays the groundwork for the next piece. Whatever it is that any of us buries deep within, out of shame, humiliation, fear, hatred -- all that stuff of secrets -- feels unique as well as burdensome. No matter how we may believe we know better, it is all but impossible to believe that the therapist has heard the same dark feelings and thoughts from others and even felt them herself.
I'm not sure I fully agree with conventional wisdom that withholding secrets and indulging in lies of omission actually impedes treatment. If the aim of treatment is the alleviation of symptoms, then yes, that is true. But if the goal of therapy is deepening one's knowledge and understanding of ones self, of getting under the symptoms to their meaning, then the struggle with lies and omissions is an integral part of the therapy, a necessary part of revealing the truth of a person's life. Ultimately, if both therapist and patient are faithful to the work they are doing, the secrets will lose their power.