tl;dr The Eurozone needs means to cope with differences in productivity and fiscal policies - either needs free floating exchange rates (ie no Euro) or it needs federal budgets and bonds (ie fiscal Union)

Either option scares the shit out of everyone so no one has brought it up.

The ex Greek finance minister thinks the current German finance minister is forcing Greece to exit the Euro so they can force the fiscal Union closer - but (implicitly) without having to messily deal with democracy or referendums.