You are being fooled about mental illness
It's not illness
It's not dysfunction
It's not biological
It's not medical
It's not chemical
It. Is. Meaningful.
At the risk of sounding overly insulting, I'm appropriating James Carville's popular 1992 phrase to emphasize that mental disorder is about the mind, not the brain or body. Carville's "it's the economy, stupid" emphasized that the economic situation during a presidential election is paramount, and far surpasses other campaign issues.
By borrowing his phrase, I'm emphasizing that the mind is paramount for understanding mental disorder. Further, not only does mind far surpass the brain and body in this understanding, the brain/body is irrelevant. The brain is not the mind and the mind is not the brain. Looking at one in order to understand the other is foolish.
Let's do a thought experiment. Take all mental and behavioral problems and identify the ones caused by defective bodily functioning: confusion brought on by drug use, uncharacteristic behaviors caused by a brain tumor, fatigue due to a vitamin B12 deficiency, mania as a result of cocaine intoxication, hallucinations because of Parkinson's disease. Those mental and behavioral problems are not examples of mental disorder. They are straightforward physiological disorders. As I explained earlier, merely having a mental or behavioral symptom does not make a problem a mental disorder.
Once those examples are thrown out, what is left? These: desiring to shut down int he face of extreme apathy and disappointment about life, a keen interest in being prepared and hypervigilant, a desire to numb painful feelings with drugs, entertaining alternative realities that better explain strange personal experiences, an unwillingness to live within social and biological limitations. These are, respectively, diagnosed as depression, anxiety, substance use, schizophrenia, and bipolar disorder. But they have no pathophysiological basis. They have a meaningful basis. They are about mind, not brain or body.
Certainly, while those "mental disorders" are occurring, the brain is undergoing physiological activity. But that brain activity is not pathological or otherwise a matter of dysfunction. It is working quite well. Instead, the problems are about how the person responds to those meaningful things and how that response works or doesn't work in their social contexts. That is the essence of mind - it is the experiential foundation upon which all meaning and choices are based. Physiologically tinkering with a person who doesn't have a physiological problem is dangerous.