Many sides have had their share in this. The Greek government of this time has made some severe errors. But I see an even bigger share in the EU (also German) politicians of that time (before the Euro was installed). It is a fact, that those politicians sold the Euro to the German population (I guess to others the same way) with the argument, that no country will ever have to pay for an other Euro country. They said, that there are so many security measures installed ...
But the weakest spot in those "security measures" where the politicians themselves. They wanted Greece to be inside the Euro, so they turned a blind eye to any warning signs. Of course they also turned a blind eye to every other country, because according to the "security measures" that where announced, nearly no country would have got the Euro ... (even Germany violated the terms)
So, I see the biggest failure in the politicians, that just wanted to have this adventure -- without really managing the risks ....
It is really an ugly business, and we will see, where all this disaster (the Euro) leads us to. In now 5 years of so-called crisis-management, they even did not manage to "rescue" Greece ... (they just prolonged the pain) and other countries can still become in trouble soon. With a bigger country, the troubles will be proliferated.
For me, the whole case is a (n other) portent of the incapacity of today's politicians.