It's interesting how quick people are to put a label on things: many put it down to 'depression' (which is somehow now a 'disease' in a fantastic butchering of language). Even the term 'hikikomori' is a nice label to stick on something that nobody really understands, so we can get these nice mental health experts in who can give us a name for something and help the problem go away. The current attempts to understand this, and similar conditions, are in my view flawed, and counter-productive. Before Japan floods itself with mental health care to 'fix the problem', I think the mental health industry needs a major self-diagnosis first.
I see such conditions as a battle between individualism and collectivism. The collective wants us to adapt to the behaviors and emotions of everyone else, which may conflict with the individual desire to be ones self (And thus, alone: the only place where we can be ourselves). Society wants conformity and familiarity out of fear of the unknown, and as such, we are quick to reject individualism, stick a label on it, and attempt to generalize so we can feel like we're back in control. The mental health industry is the embodiment of this collectivist mentality. It starts with a preconceived solution: that of 'normal', and infers the problems, giving names to conditions which lie outside of that considered normal.
If you consider that mental health conditions are never concretely defined: but instead a list of common symptoms is given a label based on statistical probability: you have the condition if 4 out of 6 of these symptoms apply, for example. The truth is this is all we understand - a very macroscopic view of behavior, emotion and chemical response from our bodies. Nobody really knows the fundamental constructs which define our behavior and feelings yet. The mental health industry doesn't let that stop it though, and is so sure of itself that it can easily apply labels and it can 'fix people', based solely on past results (which aren't that great). Ultimately, what defines whether or not an behavioral, emotional or chemical response is part of a condition is that of perspective: The perspective of those defining the labels based mostly on deviation from 'normal human'.
If we contrast the collective against the individual though, we might come to different conclusions about which needs 'treatment'. The collective favors false values like patriotism, piety and loyalty over actual moral values. It's greedy. The collective says one thing and does another. It's a serial liar. It pillages and murders. By any definition of the word, the collective is psychopathic. Yet individually, we have moral values. It's each of our fears that ultimately manifest into the collective, and the rejection of 'non-conformists' is part of that.
That's why we can have leaders, worshiped like gods, calling for the mass murder of innocent humans, and revered with medals of honor. At the other end of the spectrum, seemingly innocent young men and women who choose solitude, harming nobody but themselves and close relatives, are treated like the bane of society. They must be 'fixed'!
Japan is traditionally a very conformist society, and the pressures of the collective on the individual to succeed are massive. It's not surprising that at some point young people choose not to live for others anymore, but to be themselves - and living in the only place where they can be.
“Before you diagnose yourself with depression or low self-esteem, first make sure that you are not, in fact, just surrounded by assholes.” - W. Gibson
Also, lets not forget how often 'mentally ill' has been used as a pretext for authoritative governments to lock away dissenters for expressing their individuality: alternative views outside those deemed acceptable by the collective.