Adolescents are at increased risk for tobacco, alcohol and drug use; Peer pressure is the main reason why adolescents start using substances. At this age, the use of substances could be detrimental to the development of the brain and place them at higher risk of developing a mental disorder. Chronic disease
People living with chronic conditions like HIV and diabetes are at higher risk of developing a mental disorder. People living with diabetes experience significant stress from biological impact of the disease, which places them at risk for developing anxiety and depression. Diabetic patients also have to deal with emotional stress trying to manage the disease. Conditions like heart disease, stroke, respiratory conditions, cancer, and arthritis increase the risk of developing a mental disorder when compared to the general population. Personality traits
Risk factors for mental illness include a propensity for high neuroticism or "emotional instability". In anxiety, risk factors may include temperament and attitudes (e.g. pessimism). Causal models
Mental disorders can arise from multiple sources, and in many cases there is no single accepted or consistent cause currently established. An eclectic or pluralistic mix of models may be used to explain particular disorders. The primary paradigm of contemporary mainstream Western psychiatry is said to be the biopsychosocial model which incorporates biological, psychological and social factors, although this may not always be applied in practice. Biological psychiatry follows a biomedical model where many mental disorders are conceptualized as disorders of brain circuits likely caused by developmental processes shaped by a complex interplay of genetics and experience.
Although "nervous breakdown" is not rigorously defined, surveys of laypersons suggest that the term refers to a specific acute time-limited reactive disorder, involving symptoms such as anxiety or depression, usually precipitated by external stressors. Many health experts today refer to a nervous breakdown as a mental health crisis. Nervous illness
Additionally to the concept of mental disorder, some people have argued for a return to the old-fashioned concept of nervous illness. In How Everyone Became Depressed: The Rise and Fall of the Nervous Breakdown (2013), Edward Shorter, a professor of psychiatry and the history of medicine, says:

About half of them are depressed. Or at least that is the diagnosis that they got when they were put on antidepressants. ... They go to work but they are unhappy and uncomfortable; they are somewhat anxious; they are tired; they have various physical pains—and they tend to obsess about the whole business. There is a term for what they have, and it is a good old-fashioned term that has gone out of use. They have nerves or a nervous illness. It is an illness not just of mind or brain, but a disorder of the entire body. ... We have a package here of five symptoms—mild depression, some anxiety, fatigue, somatic pains, and obsessive thinking. ... We have had nervous illness for centuries. When you are too nervous to function ... it is a nervous breakdown. But that term has vanished from medicine, although not from the way we speak.... The nervous patients of yesteryear are the depressives of today.