He explains it very well in a single paragraph:
The second strategy, which has dominated U.S. policy in recent years, is lowering relevant interest rates and capital costs as much as possible and relying on regulatory policies to ensure financial stability. No doubt the economy is far healthier now than it would have been in the absence of these measures. But a growth strategy that relies on interest rates significantly below growth rates for long periods virtually ensures the emergence of substantial financial bubbles and dangerous buildups in leverage. The idea that regulation can allow the growth benefits of easy credit to come without cost is a chimera. The increases in asset values and increased ability to borrow that stimulate the economy are the proper concern of prudent regulation.
Note that “build-ups in leverage” will do enough of the work of the argument if you are allergic to citation of “bubbles.”