What makes the European monetary union special is that it combines a single monetary policy with national fiscal policies. The monetary policy for the 17 countries of the euro area is decided by the Governing Council of the ECB in Frankfurt.
However, the fiscal policies of the 17 members of the euro area are decided by the national governments and parliaments – each country decides itself on its own government revenue and expenditure. With such lopsided responsibilities, the individual countries have incentives to run up debt.