In short:  The Eurozone is an asymmetric monetary union, binding together rich national economies with large market-dominating oligopolies and poorer nations with smaller scale, less efficient production and weaker governance. Trade deficits between the two are inevitable.

As a consequence, surplus money piles up in the richer nations and the price of money there, interest rates, falls. Money (debt) is offered to poorer nations to access these higher returns without regard to long run risk. Eventually, debt repayment falters. And since today’s Europe refuses to consider private debt restructuring (ie losses for rich-nation banks) extend-and-pretend bailouts began, flowing straight from Greece to the foreign banks.

Read the full explanation here; you’ll need to watch the video link at the top to hear Varoufakis’ alternative policies.