The japanese debt hit 247% of the GDP, here are the main reasons :
- Too many old people,
- Very low fertility rate,
- Japanese population is declining since 2010,
- Not enough immigrants,
- Too much taxes,
- Too much fiscal stimulus (public spending, quantitative easing, ...),
- Debt own at 90% by japanese (less obligations from the government),
- More ederly people there is, more funds will be withdrawn from the hedge funds that finance the debt, and more complicated it will be to sustain the debt,
- Fewer are the young people, less will be the savings to back the debt,
- GDP not growing fast enough.
With the purchase of its own debt (through bonds), the government is responsible of a weak Yen. Which is good for large companies that export their products like Toyota, but very bad for small and average companies that get an increase of their costs.
More : http://www.bloomberg.com/graphics/2016-japan-economy/