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?! :)

Genetics, environmental factors, and bad experiences that people might have been through

sadness simple as that and nothing else   

The causes of mental illness are numerous and interdependent environment that causes postpartum Kasabh for example, or addictive and causes hereditary breed with direct human or have a ready and get larger if enacted