This isn't so uncommon. Yes, the manic-pixie-dream-girl trope is stupid, but is it worth getting angry about? I think it was just saying that, if you're going through a rough patch, if you stick with it, people may surprise you.
I've known people with real depression, and it's not something they can control. Telling them they have no right to feel that way will only make matters worse, because they'll feel guilty about something they can't control. They probably already feel alienated as it is. Clinical Depression isn't the same as feeling sad. It's a real mental condition in itself, and in my opinion they should give it an entirely different name, because "depression" isn't really an accurate term. It suggests that it's a mood that can be controlled, and it's not. To the above ass-holes who think people should just stop complaining, stop being part of the problem.
Also, the world isn't as dark as people think. Yes, there are problems, but there's a lot of good too. We're living in the least violent time in world history. And thwarted suicides happen all the time. Heck, I read an old Cracked article about an old man who walks by a popular suicide spot every day to make sure no ones there. He's saved dozens of people. Sometime he doesn't succeed, but he keeps going out there anyway. Our perceptions are influenced by our experiences, which is why it can be hard to see the good amongst all the bad.