When a person’s external reality is full of actual dangers, and they have experienced material (e.g., money) and social losses (e.g., breakup of a relationship) they are depressed and anxious for a reason. They are not feeling these things because they were mentally ill before conditions went bad. But the stress of the reality can lead to mental illness. Antidepressants work relatively more effectively when the source of depression is due to misinterpreting reality. For example, imagining conditions as negative when they are really positive. When situations are objectively problematic the drugs don’t work as well.
Today, of course, worldly conditions are very bad. Threats to health, wealth, and government incompetence are objective facts. This leads to understandable sadness and anxiety. When these mental conditions become chronic they may qualify as mental illness. And, if any illnesses, mental or physical occur, one’s ability to cope with external problems, to change conditions in the world becomes more difficult. It is a vicious cycle.
How to end vicious cycles? Obviously, reducing the external stressors. And, building up psychological resilience. This can give one the ability to cope with or remove the external stressors.