I seem to recall Matt Yglesias describing Greece's threat to leave the Euro as being something like a guy in your living room with a pistol threatening to blow his own brains out. Not too scary, but you really don't want to deal with the mess.
If so, the case of Spain is more like a guy in your living room with ten sticks of dynamite strapped to his chest. If he blows himself up, the house is coming down.
That's the threat Germany and the rest of the North face.