There were some interesting responses to my post on Japan. If you recall, there is an aversion to relationships and to sex amongst a surprisingly large number of young Japanese. My speculative answer to this (writing as an outsider) is that the underlying problem is one of individualisation.
That might seem an unusual answer as the Japanese are better known for conformism and for upholding a communal identity than they are for going it alone.
I'll begin briefly with terminology. I hold "individuality" to be a good thing, as something that reflects the natural impulse that humans have toward a creative unfolding of self. But "individualisation" I hold to be one of the worst things, as perhaps the very thing that traditionalists are most set against.
Individualisation means that even when people live in families or nations that they seek their fulfilment not together but separately, as discrete individuals. A society might, for instance, have begun with a strong culture of family life, in which people found their life together as a family rewarding, and felt closely bonded to each other, and sought rewards and fulfilments in life through their roles within the family. Individualisation would mean that this culture of family life grew weaker, to the point that the family had less purchase on people and meant less to them, so that they instead sought their fulfilment alone as individuals outside of the family, even if they still lived under the same roof.
Individualisation can come about through the influence of value systems (for instance, through theories about personal freedom or the expression of individuality). But it can also come about when the framework of a society no longer fits together well - and it's my theory that this might have happened in Japan.
As I mentioned in my first post, when I lived in Japan my male work colleagues had a punishing lifestyle that meant that they were rarely home during the week. They were all absentee fathers, at least until the weekend. I suspect that this lifestyle required strong beliefs in masculinity and in national identity to be sustained ("this is what Japanese men do"); similarly their wives must have acted from a sense of what was expected from their own role.
But the cracks have now appeared. For some younger Japanese men, masculinity is something that appears oppressive, rather than fulfilling; similarly there are young Japanese women who prefer the idea of remaining an independent single girl to marriage. For many young Japanese the bar was set too high.
How else can individualisation be fostered? If marriage is delayed for too long, then individuals face a dilemma. If you're a man, for instance, and you're not expected to marry until you're 35, then it becomes difficult to look to family and to family roles for your fulfilment in life; you have to think, instead, of how you can seek fulfilment outside of such relationships.
Maybe too there's a problem if there is an excessive sexualisation of very young women. If young men at a critical point in their development observe the women around them being very casual in their sexual mores, then it will be more difficult for them to imagine a commitment to intimacy with such women.
What a society needs to do is to understand what it is that draws family life apart (in the direction of individualisation) and what it is that brings it closer together. I'm not sure that there is a very honest attempt to do this in the modern world. In most of the articles I've read about Japan the preferred solution is to better integrate women into the workforce, but this is part of a drive to utilise women as a labour resource and to promote female independence rather than a serious reflection on how to uphold a culture of family life.