Most Helpful Girl
It's called expressions. They show what people feel. So it sort of is self explanatory that someone mentally ill most often than not cannot school or portray their features or hide using them.
That's sort of like asking why some happy people smile.
Most Helpful Guys
I'm not an expert on psychology, but it could be because they have a different way of looking at the world. The reason they're mentally ill is basically because there's something wrong in their thinking. And it's our thoughts that show up as expressions on our faces. The "special" look in a mentally ill person's eyes could be a result of his ill thinking.
That face is usually a mix of all emotions coursing theough them and their neural system is not wired in coordination with the rest of their bodies so sometimes they can't control their facial expressions and that's why they have that certain look on their face...