North Korea only exists as a sovereign state because China allowed them to do so.

NK acted as a buffer state between China and the US backed states of South Korea and Japan.  The twentieth century history of that area has brought us to this situation where it is now difficult for China and almost impossible for the US to do anything to curtail the ambitions of the NK leadership.

Looked at from their point of view you can understand why they are so keen to have a nuclear capability - without it their future as an independent country looks very doubtful.