Michael Pettis:

just got back from a very interesting but hectic week in New York and Washington, followed by two days at a conference in Hangzhou. During my meetings I noticed that much of the discussion, and many of the questions I was asked by both government officials and investors, focused on debt levels and reforms in the Chinese financial system. I have written a lot about rising debt in China and am glad that analysts and policymakers seem to be spending a lot more time thinking about balance sheet issues. Every case of rapid, investment-driven growth in the past century, as far as I can make out, has at some point reached a stage in which debt levels rose to unsustainable levels and precipitated either a debt crisis or a long grinding adjustment period.
The reason debt levels always seem to grow unsustainably, I suspect, is that in the initial stages of the growth model much if not all of the investment is economically viable as it pours into building necessary infrastructure whose profits and externalities exceed the cost of the investment. The result is real growth. At some point, however, the combination of subsidies, distorted incentives (in which investment benefits accrue to those making the investment while costs are shared broadly through the banking system), and very cheap financing costs leads inexorably to wasted investment and debt rising faster than asset values. This is when the debt burden begins to rise in an unsustainable way.