I have boiled down the root causes of mental illness to 3 things; feedback and comments/challenges are encouraged.
1) Desire (I'll go with the Buddha on this one)
2) Attempting to control things that are beyond one's control (an offshoot of desire)
3) Identification with a false concept of identity.
Brief explanations:
1) Desire: Whenever a person has any type of desire, it implies that they are dissatisfied with what currently 'is.' Dissatisfaction implies discontent and a lack of happiness. If you want something that you don't currently have, this is a problem.
2) Attempting to control things that are beyond one's control: This is one of the leading causes of anxiety, anger, etc. How often do we define our own happiness according to the actions of other people, communities, governments, girlfriends/boyfriends, husbands/wives, etc.? How often do we become frustrated when our attempts to change these people, communities, governments, etc. fail?
3) Identification with a false concept of identity: Who are you? How did you reach that conclusion? According to all 11 definitions of identity in Webster's Dictionary, identity implies stability over time. Yet, how often do we identity/define ourselves conditionally? For example, let's say that someone says, "I am a teacher." Ok, great. Now, if your job is in jeopardy, then your identity is also in jeopardy! Now, in contrast, how many would have answered this question by saying "I am an observer"? For, as long as we live, we observe.
Note: I would expect one of the most common challenges to these assertions would be, "Well, what about chemical imbalances? What about genetic predispositions?" To this, I would remind everyone that environment vs. genes (i.e. nature vs. nurture) is a false dichotomy. It is known scientifically that interactions with our environment has effects on the genome which are then passed down and inherited generation by generation. Thus, I would assert that any 'genetic predispositions' for a mental illness are the results of the 3 root causes that I listed to begin with, but in previous generations.
Basically ignorance is the root of all mental illness.
The Tibetan (Buddhism) philosophy has it right in my opinion as they acknowledge that their is two truths (but only one is the real truth, just stay with me).
The fake truth is when you watch tv you see a silly cartoon cat chasing a mouse.
The ultimate and real truth is you are viewing tiny dots of light.
So I say ignorance is the root of all mental illness because its ignorant to think one exists at all or even acknowledge an "I", "me" or "you" unless they are talking about the ultimate one "I", the infinite self.