Then you stop it. Go ahead, we'll wait here.
Climb off your high horse. It sure as hell isn't as easy as you make it sound. It's not like turning off a valve. Human beings are incredibly complex, both psychologically and physiologically. There is no wonder cure. It's probably always going to exist as long as people have emotions, to be honest, and our mental states are ruled by chemical processes.
No, it's not like turning off a valve, but recognizing that there is a problem and talking about it - even if it is on the internet - sure helps.
Ultimately, a person only has control over themselves. The best you can do is try to convince them see things differently. Even then, we all have our own demons, our own problems, our own tragedies, and there certainly isn't a magic fix for all of them.
Idk if it was your intention, but it really felt like you were saying that people have depression because they don't want to see the good in the world. It's not about "convincing them to see things differently". They want to see things differently, they just can't.
Everyone has their own demons, but the demons look much scarier for people with mental disorders.