[foobar2020]

It is true that a mental patient can be forced to treatment only if (s)he is considered a serious danger to the society, or herself/himself (includes suicidal tendencies). In many jurisdictions a court's decision is necessary, usually obtained on a rather short notice (24-48 hours).

But this does not mean that patient's consent is required for a valid diagnosis of a mental disorder. Denial is a serious problem and somehow an internal part of certain diseases such as mania, or, as you mentioned, early stage addictions.