Your second sentence where you try to say people with depression would be depressed even if everything was right in their life is completely unfounded by research and literally just not true. Symptoms of depression are known to improve in many instances where life circumstances improve. The whole point of talk therapy is being able to challenge dysfunctional beliefs, which can improve symptoms of depression. Sounds like you just pulled that out of your ass. Anyone who’s taken a psychology class knows that scientists don’t even have a solid idea of what exactly causes mental illness, and not only that, but the definitions and diagnostic criteria change each time the DSM is updated. “That’s what makes it mental illness and not just being sad.” That is completely false. The difference between “just being sad” and having depression is dependent on the level at which the symptoms are impairing your function as well as the duration and consistency. A person with depression could see a major improvement in symptoms if there is a change in quality of life or if they gain the tools to better manage their symptoms.