Evan Feigenbaum has a theory:
Beijing rarely shares American threat assessments. And China’s leaders, even when they do sense a challenge to “stability,” are far more relaxed than are Americans about the scope and nature of those threats. This is certainly true of Pakistan, where Beijing trusts the military’s instincts and senses little threat to the Pakistani state. It’s true of Iran. And it’s true of North Korea, which few Chinese believe will collapse and where a managed transition toward Chinese-style reform is the medium-term outcome China seeks to achieve.
(Hat tip: Greg Scoblete)