The German banks made loans to the Greek government for the construction of among other things, the Olympic park and the subway system, which were built at enormous profit by German engineering and construction firms.
Those loans, and the terms under which they were granted, were not economically sound. What happens when a bank makes a bad loan is that they take a loss on the principal and restructure the remaining principle under terms that are feasible. This is what the Germans REFUSED to do, instead, nationalizing the bad debt by taking it out of the German banking system and placing it in the European Central Bank
Feasible terms? The private and bank holders took something like a 50% haircut. When the ECB, IMF and Euro governments took over the remainder, they stretched out the due dates to something like 30 years at an interest rate around 2%.
If the Greeks can't handle that, it might be related to their government spending close to 60% of GDP.