I found this extraordinary citation from John Hussman's newsletter in this article at Seeking Alpha and will simply repeat it without comment:
"The basic problem is that Greece has insufficient economic growth, enormous deficits (nearly 14% of GDP), a heavy existing debt burden as a proportion of GDP (over 120%), accruing at high interest rates (about 8%), payable in a currency that it is unable to devalue. This creates a violation of what economists call the "transversality" or "no-Ponzi" condition. In order to credibly pay debt off, the debt has to have a well-defined present value (technically, the present value of the future debt should vanish if you look far enough into the future).
"Without the transversality condition, the price of a security can be anything investors like. However arbitrary that price is, investors may be able to keep the asset on an upward path for some period of time, but the price will gradually bear less and less relation to the actual cash flows that will be delivered. At some point, the only reason to hold the asset will be the expectation of selling it to somebody else, even though it won't be delivering enough payments to justify the price.
"Unless Greece implements enormous fiscal austerity, its debt will grow faster than the rate that investors use to discount it back to present value. Moreover, to bail out Greece for anything more than a short period of time, the rules of the game would have to be changed to allow for much larger budget deficits than those originally agreed upon in the Maastricht Treaty."
