Mental illness is different from other physical illnesses in that there is no single instrument or test that can confirm the diagnosis. Almost all psychiatric disorders are diagnosed based on clinical symptoms and after excluding other diseases. Therefore, many patients and their families have the question, “Why would a patient have ‘mental illness’ when all the tests are normal? So, what is the cause of “mental illness” and is the cause the same for different types of “mental illness”? The current medical understanding of the etiology of psychiatric disorders is briefly described. Understanding of central nervous system mediators. Many studies and surveys have found that almost all psychiatric disorders, including schizophrenia and depression, are associated with disorders of central nervous system mediators, including dopamine, pentazocine, norepinephrine, glutamate, gamma-aminobutyric acid, acetylcholine, and many others. The therapeutic mechanism of most antipsychotic drugs is based on the modulation of central neuromediators. The reasons for the occurrence of disorders of central neuromediators are still unclear. The understanding of the structure of the central nervous system. Patients with Alzheimer’s disease (the most common form of dementia) have extensive brain atrophy, most pronounced in the temporal, parietal, and prefrontal lobes. Vascular dementia, mental disorders due to diffuse sclerosis, mental disorders due to CNS infections, mental disorders due to craniocerebral injury, etc., all have clear CNS pathology. The influence of somatic diseases. In the course of endocrine, nutritional, metabolic, hematological and other systemic diseases, mental disorders arise due to the impact on brain function. The psychological reaction of the patient to the somatic disease, together with the biological factors generated by the disease, leads to the development of psychiatric symptoms. In most cases, it is often difficult to distinguish between the two aforementioned factors. Genetic basis of psychiatric disorders. Psychiatric disorders have a tendency to cluster in families. In addition to being genetically related, risk factors such as transmission of upbringing, pathogenic behaviors, and lifestyles are carried over from one generation to the next in the family through learning or imitation, or increase the risk of morbidity due to exposure of family members to common environmental factors. As with many disorders, most psychiatric disorders are polygenic. There are also a few disorders that are closely related to psychiatric disorders that are monogenic, such as Huntington’s disease and hepatomegaly. In addition, the risk of disease in children is greatly increased when consanguineous marriages take place. Psychosocial factors and mental health. Benign stress can make people happy, enhance motivation, and promote growth and development. Adverse stress mostly refers to more serious, extended and out-of-control stress states, which, if not dealt with in a timely manner, can lead to physiological function or metabolic disorders and tissue junction damage in the organism, and even lead to the occurrence of disease. Various important events in life can be considered as stressors, but the response to stress is obviously influenced by individual characteristics (personality traits, cultural qualities, previous experiences, etc.) and varies greatly. Culture is created by the perceptions, passions and will of people. Culture contains and reflects the psychological characteristics of social groups or nations, and the inner world of individuals is clearly influenced by a specific culture to form psychological traits and behavior patterns. The manifestations of some mental and psychological symptoms are closely related to the cultural milieu to which they belong. Research in the field of sociology and anthropology has been intensifying, and the interrelationship with mental illness is gradually being emphasized. Various theoretical schools of psychology have different views on the emergence of mental illness. The psychoanalytic school emphasizes psychodynamics and early developmental experiences; the behavioral school considers abnormal behavior as a type of learned response; the cognitive theory emphasizes “inappropriate expectations”; and so on. Psychoactive and non-addictive substances and mental health. This point has been increasingly taken into account by medical and non-medical practitioners. In the case of alcohol, for example, the World Health Organization (WHO) noted in 2004 that harmful use of alcohol is one of the most serious health risks worldwide. At the same time, there has been a dramatic increase in alcohol-related social damage (e.g., drunk driving leading to car accidents). The prevalence of alcohol-related psychoneurological disorders in China is far from the recognition rate, and education and publicity of related knowledge should be strengthened. Alcohol-related psychoneurological disorders mainly include acute alcohol intoxication, alcohol withdrawal symptoms, and alcohol-induced psychiatric disorders. The development of mental illness cannot be the result of a single factor, and even for a particular patient, it is impossible for a doctor to fully explain the cause of his or her illness.