Bankers and other financial types invested money they would not otherwise have invested in a dodgy creditor (Greece) in the mistaken belief that the EU/ECB was obliged to bail them out.
When Greece had trouble re-paying the debt the creditors should have taken a haircut (re-negotiate the loan so that most but not all the debt could be repaid).
But wait a minute most of the financial institutions that made the wrong call were based in France and Germany -- so suddenly the EU/ECB was obliged to mount a rescue; Not of Greece, but the idiots who had lent the money to a country that was clearly incapable of paying it back.
Having successfully saved it biggest banks and pension funds the German government balked at actually saving Greece (or even funding the EU bailout which saved their precarious banking system).
So the richest country in the EU, got bailed out at the expense of the poorest country. Then complained about it.
If we are all going to get to the end of the 21st century as anything but subsistence farmers then we really need to find an alternative this antiquated Victorian financial system which has reliably collapsed every 10 or 20 years for the past two centuries.