Why is it so hard to get a nuclear agreement with North Korea?

The negotiations have been going on for years with nothing to show. The North Koreans would like the sanctions lifted and the rest of the world would like to see NK denuclearize.

You start on a small scale – sanctions lifted on some goods in exchange for destruction of some weapons, witnessed by outside observers. The initial step can be modest, just to get things started. If things work out then hold another meeting. Lift more sanctions in exchange for the destruction of more weapons. Keep doing this until there are no more weapons and no more sanctions. Along the way, the Korean War can be brought to an official close.