Japans a bit weird in that their debt is larger than their GDP by a significant amount but people trust that their money will not be lost or stolen in the system or the currency will collapse.
Benefits of having your own citizens holding the debt still to get ROI would be a significant challenge for them and they can't really print money to increase their debt so its a bit of a conundrum.
Debt larger than GDP is fine. A more meaningful measurement would be the country income vs debt level. So long as the country has good income via export and service, the debt can be expected to be paid back.
I should have clarified a bit more on that
Here is a neat info graphic on what I meant in relative terms
http://demonocracy.info/infographics/usa/world_debt/world_debt.htmlTotal debt ratio is about 250% to GDP but your right Income to Debt Repayment is around 55% and Interest is 2% of their debt ratio to GDP so its not that bad although I could point out Italy looks healthier based on that data so in part I guess its how strong you are in the world economy as well.