This makes sense in the case where the patient is asking for help and both the doctor and the patient agree on the best course of actions. But much more often the case is that the patient is desperately in need of help and the doctor is coercing/convincing them of a treatment they think is best for them -- so the ground is not so even.
It may be subtle to most and unobvious at first, but say you consider this:
The patient has been through some psyche/soul shattering experiences, and their body is trying to recover from and avoid further damage from that impact with what seem to us unproductive processes, such as in the case of PTSD related actions/reactions.
The patient will seek help in coping with their body when it comes to these behaviours because in human society they are perceived as unproductive. This doesn't mean, however, that they are useless to the patient's body (themself).
The doctor will assess the patient's behaviour based on current human social norms, and how the "illness" is affecting their daily life. Then, they will proceed to convince the patient of a treatment the doctor thinks will work to reduce the impact of their "illness", thinking that this will have "cured" them.
But what if all this accomplishes is to suppress certain natural processes in the body, and as a result, the patient will not live a healthier life from these treatments, even if they appear to help return their human life "back to normal" somewhat. The treatment(s) will, in fact, cause further stress on the body -- as is often the documented case with so many side-effects.
Then, to complicate this, add the fact that mental illness has a strong stigma attached to it in our society, where mentally ill people are considered less that reasonable, and their judgement impaired. In instances like this, especially if the patient is a perceived threat to others or to themselves, the doctor gains power over the patient's own free will, deciding entirely what is best for them and their body.
There are many accounts of people getting better from the effects of their mental illness by NOT listening to their doctors, and listening to their body and learning to work with their body instead. After all, doesn't the patient know their own ailment best? How can a doctor claim to know someone else's body workings better than they do themself?
As a patient, this puts a big moral question mark on the whole mental health and medical industry for me.