Money. More than likely it revolves around money.
http://oudaily.com/news/2013/feb/04/north_korea/
“Past negotiations for food and other forms of aid from the U.S. have worked as an extortion racket. North Korea promises to stop developing nuclear weapons and the U.S. gives aid as an incentive. Then, North Korea resumes developing nuclear weapons and the process starts over again. Extortion is usually the powerful extorting money from the weak, but thanks to leadership in Washington, North Korea is somehow extorting us.”