Is there a sociological reason why Japanese people are not having children at rates that sustain the population?
Worse, I was watching the Japanese news (~6 years ago) which had a discussion where an economist pointed out that while wages have stagnated since the late 1980's, base taxes have gone up from 2.5% to 22%. The younger generations now have elder parents asking why they aren't being taken on trips around the world like they supported their parents, and the answer is simple: no one can afford it, let alone afford to have children.
Just my personal thoughts, this comment is not meant to include any judgement opinion.
Another interesting idea, is that over the long-term, cultures that allow their members the freedom not to have children, may be overrun peacefully or forcefully by those that don't, or that at least, provide larger incentives to have children. Moreover, again over a long enough time horizon, people's who naturally don't have the inclination to have children, will disappear from the gene pool, making having children (all other things equals) the most common and culturally sensible option.
I also know there's a tendency in more developed (or at least wealthier) countries that as they become more prosperous their populations have fewer children. I'm sure there's multiple reasons, but in the US it can be difficult to have children and support them in a single income. Yet at early ages daycare alone can eat into a significant part of someone's takehome salary. That economic equation puts some downward pressure on the # of kids.