I don't think there is a simple answer to this. Parents tend to take a "softly, softly" approach when children (especially eldest sons!) show troubled behavior -- much more than anyone would do in the West -- and there is a widespread cultural perception even now that it is a "private, family matter" rather than a problem that the police, doctors or social services should get involved with; and also that it is "a phase" that the boy will grow out of. For this reason the situation can and generally will be allowed to go on for years, where outside intervention would be called in if it were a Western country. It often seems to be triggered by bullying, which is another cultural problem in Japan. Bullying in Japanese schools goes far beyond what would ordinarily be tolerated in Western countries, as well, including organised and sustained campaigns of hate mail and physical attacks. To an unfortunate extent this is tolerated and even expected, since there is expected to be a strictly enforced "pecking order" in school classes, and those who don't "fit in" are often automatically at the bottom. However, even the Japanese have difficulty figuring out all the factors that go into it. As well as the contributing factors above, partly it is the reluctance of courts to get involved, partly it is a traditional Japanese glorification of the "nobility" of solitude, which makes locking oneself away more socially acceptable, and partly it is a reluctance to regard it as mental illness, as mental illness is deeply stigmatized in Japan and parents may well regard mental illness as something which brings deep shame on the entire family. This may keep it from being dealt with directly or effectively.

There is an element to Japanese culture which runs through nearly all organizations and institutions here; Bullying!! The pressure to conform to the group, or its leader(s) starts at an early age and continues relentlessly. An increasing number of young people find it too much to cope with, (especially those who stand out for whatever reason) It can be quite a shock to outsiders who come to Japan, to see just how militarized the school system is. Even kindergartens can have an element of "Boot camp" to them. And although largely frowned upon by society, the tradition of buylling and establishing a harsh pecking order is still very common. Hikikomori and youth suicides are but two of the unfortunate byproducts of this.