Not being an expert in this matter all I can give you is possibilities and/or my opinion.
While I have suffered a brain injury in the past and have recovered 95% of what I had lost I can only imagine the horror of what it would be like to be stuck feeling nothing but negative emotions or even have no control over your emotions.
With that being said there is always a spiritual side - to everything. As mere humans we are not often privey to such details. Some possibilities are as follows....
1) The soul/spirit of the indvidual might need to learn something by experiencing this condition.
2) The soul/spirit my have volunteered to experience this ordeal in order to teach something to one or more individuals.
3) To help advance scientific understanding of the human brain.
4) To motivate a certian individual(s) to persue learning alternative healing methods.
These are just the first things to pop into my mind. You just have to keep in mind that there are no accidents, everything happens for a reason. And because everything happens for a reason these brave souls deserve our respect and compassion.
If you look back in history people with mental illnesses were more often than not treated cruelly. They were often beaten and tortured with such sadistic brutality it would make most people today physically sick. But look at how things have changed. Our system today of treating the mentally ill may not be perfect but, it is much better than it once was.
Well, these are just my thoughts upon this matter. Hope they help.