Because the euro isn't just a currency, it's the entire banking system as well. The euro doesn't work as a currency without a banking system to transmit it beca

Because the euro isn't just a currency, it's the entire banking system as well. The euro doesn't work as a currency without a banking system to transmit it because people have bank accounts with banks... not the central bank. If a bunch of German/French banks suddenly get 40billion in losses what happens next? They will have to panic sell their remaining good assets to remain solvent, that transmits the problem on to the other banks that they have invested in. Because the EU banks all tended to lend money to each other. So you end up with the problem transmitting around europe (and beyond) but it's worse because everyone is trying to sell everything at the same time to remain solvent.