In short -
Greece national debt was not consolidated when they joined the Euro and Greek govt. loans simply converted to Euros based based on Exchange rate then and transferred to banks.
Euro then rose in value wrecking the paying back of interest + principle by Greek govt. When economy turned down servicing of this debt became impossible, leading to default.
Lesson for others
Your rulers and finance ministers should KNOW how economy/society functions and should not be ONLY lawyers writing laws to change economy/society laws hoping magically everybody will follow them and everything will fall in place.
we have been warning of this since April.(The recent bailout talks were a farce)
http://asian-defence-news.blogspot.in/2015/04/greek-default-nears.html
http://asian-defence-news.blogspot.in/2015/04/unstable-europe-not-good-for-global.html