My own view is that Mental illness covers a huge spectrum of disorders/problems from anxiety based issues that can often be overcome by relaxation and cognitive behavioural therapy and correct spiritual grounding to problems caused by childhood attachments and deep seated abuse to the major illness such as bipolar and schizophrenia.

Some of these disorders may be as utterly physical as say heart disease. Some people are born with a tendancy towards heart disease, and unhealthy lifestyle can trigger it, but there are some people who do everything healthily and still develop it...

Schizophrenia is a physical condition, but of course if you smoke weed, you increase your of developing it 12 fold if vulnerable, however, sadly there are people, through no fault of their own develop it.