It's not so much a banned topic but it's a topic that most people don't really know about. All they have is this blurry preconceived very negative stereotype of what it's about, and so everyone prefers to talk about 'happier' things, no one wants to go 'there'. Until one day after many years of inner suffering they decide to summon up the courage and ask the doctor why they keep and the doctor tells them they have depression, they have this or that. And then suddenly they become one of the people asking why no one wants to talk about mental illness! That's it.