I have this hypothesis that many of these so-called mental disorders are simply natural human traits that happen to be incompatible with our technologically-dominant world.
Man's primal instict is to survive. In an age of hunting and gathering, the way to do that is simple: find food and water, and avoid predators and danger. You can see your goals right in front of you, and you understand why you need reach them, so it's easy to move forward.
But in our world now, things aren't as straightforward anymore. To survive, you need money to buy food, and to earn money you need a job, and to earn a job you need skills that you might have to learn at school, which needs more money, and you also need money to pay taxes, and at your job you'll likely be doing something very unrelated to your instict to survive.
And so we get these people who cannot function as "regular" members of the society. As humans, they still have the instinct to survive, but they can't connect that to the things they have to do for it.
It's survival of the fittest, and though it's sad, this means that these people who can't adapt to modern society are the weak who should naturally die off. No one is at fault, they just happened to be born during the wrong stage in humanity's existence.
It's like being a lab animal, starving and just lying still, after unsuccessfully trying to grab the food that's just out of reach past the bars of his cell, while the button to move the food closer is just beside him.