Actually this phenomenon isn't that foreign. Less children being born, a falling number of "serious" relationships etc. are all things that can be observed in any technologically advanced society.
Japan is just an extreme case because their current set of values doesn't deviate as much from more traditional values, which escalates the problem.
Germany is actually somewhat similiar to Japan when it comes to these issues and while the results are by far not as extreme the reasons are very similar: lack of government support for modern types of families and traditional values that still have a strong impact on how children are supposed to be raised.
Japan is just an extreme case because their current set of values doesn't deviate as much from more traditional values, which escalates the problem.
Germany is actually somewhat similiar to Japan when it comes to these issues and while the results are by far not as extreme the reasons are very similar: lack of government support for modern types of families and traditional values that still have a strong impact on how children are supposed to be raised.