You probably mean all the 'bad' things in life with 'mental illnes, evil, clef chin,...'
If not, then read the first comment. Mental illness is not the same (at all) as 'Evil'
If yes, then I would say that there is indeed a reason for it.
It may come around as cliché, but; without any bad things, there wouldn't be any good things.
Bad things and good things are here for us because we interpretate things as 'bad' or 'good', we are human. We are born with ethics. With opinions, with preferences. So of course there is, for every individual a reason for all those bad (and thus, good) things.
It just depends on an individual what kind of ethics you have, what kind of moral or preferences you have.
It is no good dwelling too much on this question. It will drive you insane, really. Before you know it, you think of God (no offense to believers tho)
You're lumping mental illness together with evil.
I'll start with "Evil". Someone has an interest in some outcome, perhaps for the sake of experiencing certain emotions or perhaps for some more tangible reason like making money, and the means to that end involves hurting other people. Pretty easy explanation.
As for "mental illness" that is the result of the drive of people to categorize people's minds and behaviors. Really it all boils down to behavior because something must at least exist as a potential behavior for the mental experience to be accessible to others. And even if we could access more there'd be no point other than entertainment to look at parts of the mind that have no relevance for behavior.
Even when the mind involves observable behavior we're still just making educated guesses even psychologists are just making educated guesses. What if someone loves pretending to hallucinate and will never give away that that's all they are doing because they love the attention? What if that person had been a psychiatrist himself and learned how to put on a good act from their patients?
We don't even really know whether others have minds or not. Some would say that regardless we act like it but what is it to act like others have minds? I see no reason for any difference in my behavior whether others truly have minds or are just "philosophical zombies". My best interests are met with the same courses of action regardless, so the question of whether others have minds or not is an amusing philosophical topic but has nothing pertinent to add as far as the best actions to take. This is similar to question of whether this world is real or an illusion of some sort. Regardless I'm here interacting in it so any goals I have or desires I have must be met within it, so once again irrelevant to determining the best behavior.
This doesn't mean the concept of "mental illness" is useless but rather than thinking of it as something real the concept should be thought of as a useful tool. People (psychological researchers) observe and document dysfunctional behaviors, they observe what seems to work in helping to change it or at least to refocus it and what doesn't and what side effects are had for both therapy involving drugs and not involving drugs. Mental illness can not be reduced to biology, at least not biology within the realm of human observation (if one is a materialist one could argue that it all must have to do with physics and any physics involved is biology but there's still the fact that not every aspect can be observed and mainstream physics agrees that we will never be able to observe everything in physics no matter how hard we try). Biology shows correlations and that aids in treatment but that doesn't mean the biology is the mental illness. There are always exceptions. Rarely do you see a 100% correlation.
If there was a good reason then the act wouldnt be considered to be bad. So christians who say everything happens for a reason are saying evil doesnt even exist. If everything happens for a good reason then evil is no different than goodness. They are both happening to bring about the best possible outcome or whatever. Saying that god allows evil for a good reason is ridiculous. If he cant accomplish the same thing without us having to suffer then he isnt all powerful. So the only reason we could suffer with an all powerful is because the god wants us to suffer. But when you look at things they seem random. If we all suffered for a reason we would all go through the same things but some suffer greatly and dont hardly suffer at all and there is everything in between. And everything else is like that. And im supposed to believe everything happens for a reason when everything thats possible happens in most cases. Where we are born, who our parents are how many siblings, all of this stuff has to be random. It cannot happen for a reason because too much stuff happens. I can assure you that not everything thats possible happens for a reason. That leaves no room in existence for bad.