Japan is an island nation without much in the way of arable land or other natural resources.
Supporting their current population (~127 million) requires them to import nearly 60% of all their food and ~85% of all their oil and gas...this is why the Japanese government has been so laser-focused on reducing energy use (and why they're so dependent on nuclear power).
Japan's population falling is a good thing. Rapid population growth (and desperation to acquire the resources necessary to support that growing population) is what drove their colonialist impulse in WWII...population reduction for Japan means a better chance for long-term political stability in East Asia and a more sustainable Japan in general.
I'm of the opinion that ultimately, human beings react to the environmental pressures that they're subjected to...Japan is expensive and crowded, so it makes perfect sense that the Japanese would have less children (or forego having children altogether). As the population decline continues, at some point those pressures lessen (real estate costs, food costs, energy costs, etc.), and we'll see the population stabilize at a new equilibrium.
All the cultural changes are interesting, but I don't think they're the cause of the population decline...instead, I think those cultural changes are a byproduct of the environmental pressures I listed above.
Long story short, the sky isn't falling.