Because the cohesion of the euro area depends on all countries prospering, what happens within each one is not just a national interest, it is a collective interest. Every member of the euro area has a vital interest in ensuring that its partners are meeting the membership requirements – and not just at the point of entry, but continuously.
Indeed, it is sometimes asked what differentiates the euro area from a fixed exchange rate regime, and the answer is precisely this point. In a fixed exchange rate regime issues of economic integration are dealt with as foreign affairs. In a monetary union, they are domestic issues for all.