What causes mental illness?

Unknown. But Nowadays research reveals that Family history of Psychiatric problems is one of the factors of acquiring such. Environmental and social factors can also be a causes of mental illness. Chemical Imbalance in the body due to underlying causes could lead to mental illness also.

A mental illness is a condition that impacts a person's thinking, feeling or mood may affect and his or her ability to relate to others and function on a daily basis. Each person will have different experiences, even people with the same diagnosis. (Autism,Bipolar Disorder,Borderline Personality Disorder,Depression,Dissociative Disorders,Eating Disorders,Obsessive-Compulsive Disorder,Posttraumatic Stress Disorder,Schizoaffective Disorder,Schizophrenia and so on) Are you sure you are ready for real answers?! :)

sadness simple as that and nothing else