Normally a big country's currency will appreciate once they start running prolonged large trade surpluses, but the Euro has enabled Germany to keep siphoning in wealth in a way that wasn't possible before the Euro. Similarly pre-Euro the southern countries would have seen their currencies depreciate and get a break in competitiveness.
Hartz is also part of it, Germany has kept labour costs down while southern countries in the same currency area experienced high inflation due to masses of cheap money coming in again due to the Euro.
And of course Germany has been leading the austerity brigade in response to the crisis.
http://www.brookings.edu/blogs/ben-bernanke/posts/2015/04/03...