How is that about mental illness? I genuinely don't get it. People can be unhappy without being mentally ill.
Sure. But some people feel unhappy all the time. Sometimes for years, sometimes for their entire life. In their case, it is a mental health issue. It's not something they can just shake off when they hear the age-old "you're depressed because you choose to be" argument by a smug author. This approach is outdated and it doesn't work.
Sometimes it is, not always. When I went through long periods of unhappiness it was because of my circumstances and life choices.