When I was way younger I was given a book by my psychologist. I do not remember the author or title, all I know it that I only read about 2 chapters and got scared. It basically told about how demons possess your body to form mental illnesses - what the hell was she doing giving a 16 yr old girl that book to read???
Anyways, because the human mind is a constant problem solving machine I have come up with this eerie theory. I have done no thorough research on demons, relegion or even mental illnesses. I am a psychology minor - thats it. Keep in mind I do not believe in this theory, nor do I fully disregard it. Just a thought that makes too much sense to not mention.
As much as we do not want to believe it, we must accept that there is a dark/evil presence looming around us in this world, along with a bright, happy one. The dark presence can represent all that is "bad". I do not know what shape or form it takes, but its there, and it probably preys on the "weak".
The "weak" can be used to describe drug addicts, alchoholics, the mentally ill, the soulless. Imagine if you were this demon being, who would you want to or be able to prey on - maybe someone with power to persuade others, maybe a beautiful rich person - those ARE the people society looks up to, right?
Side note - some say that mental illnesses can be hereditary. These demons have been around awhile. Perhaps they have preyed on a weak family for years and find it easier to prey on the offspring, or try to.
We are in a constant battle with these seedy lil demons. Its not scary or bad, just a part of life. You know the expression "the angel on one shoulder and the demon on the other", well its not that far fetched. And you're not a bad person if you take the easier way out. Some become stronger because of these battles, some battles are bigger than others and some will have to battle all their lives. Its part of life, you should feel pain just as you feel joy. Some may think "Oh no no, I have no split personalities, never depressed, no migraines, my life is perfect and I am happy." That may be the case, but you better watch out because that may be the worse case - an able bodied, walking, talking demon with no goodness left inside to purge it out.
Which brings me back to my theory - although demons can cause mental illness, mental illness - in turn - is a way to fight it off before it takes over completely. For example - an anxiety attack - if a demon was trying to fuck with you and the anxiety attack never happened, it may just have won that battle. Im sure its different for anyone who suffers these attacks, but it is noted that one always feels better after them - back to normal. In my theory, the demon has been purged. Migrains are a condition which can be brought on by stress (deep thoughts and stress that will not be ignored). If those feelings were ignored - the demon probably won his battle. Many times a migraine is cured with throwing up - purging that nasty thing out. Again, I write this as fact, but I myself have a hard time believing my own thoughts. Just makes too much sense to discredit it altogether.
Take for example split personalities: The demon took over a part of you - so you formed another "personality" in defense.
Mental illnesses, and there are many of them, could be formed as a defense mechanism to keep the demon out. Im not saying its a good thing to always be depressed or be in migraine pain for the rest of your life. If never cured - the demon loves it. But with these symptoms you can at least take the next steps to find a cure, and you're not an able bodied, healthy, walking, talking demon roaming the earth.