system
Given the following post, respond as if you're commenting on Reddit:
human
Why do investors buy Japanese debt even though it’s approx. 280% debt to gdp, whilst investors sold out on Greek debt when it was only at 110%?
gpt
The debt to GDP ratio is not really the main deciding factor there. 

Japan has a ultra educated population, productive working force, high consumer market, powerful geo strategic allies, low criminality, anecdotal corruption, high value exports, etc. All those factors make up a very stable economy and a very stable country. This leads investor to believe that whatever debt they buy will keep its value

Greece may be part of the EU which gave them a very stable geopolitical position at the time ( we all know how that is evolving nowadays... ), but they have no noteworthy export except tourism. Add to that hey also had officially very high unemployment rate which was mainly due to the prevalence of undeclared work, which translates to tax inefficiency and missing revenue at every level. Corruption was rampant, and the lack of control over their own currency further decreased the confidence of investors into the stability of Greece and it's response to an eventual crisis.

The crisis did happen, confidence was shaken, some started to sell, and history happened.

Basically, it all boils down to confidence, which is why investors might buy Germany debts even with a negative interest rate : it's just so stable that overtime it is a good investment to safeguard your assets.