The reason for North Korea's nuclear development is for communist unification, so called "Juche Revolution" as recorded in the North Korean Constitution. The regime had never abandoned communist unification since its establishment in 1948. North Korea broke out the Korean War in the June 25 in 1950 for the purpose of it, and thereafter have provoked numerous limited warfare including the 425,571 violations of armistice agreements.
The communist unification called as "Juche Revolution" is the goal of North Korean regime and appears in every North Korean official documents and events first of all, including of the North Korean Constitution as well as the Covenant of the Workers' Party of Korea, which rules North Korea. The regime has developed nuclear weapons for the past 70 year since the establishment of the Institute of Nuclear Physics in 1954. The dictators in Pyoungyang openly have insisted that they can't give up nuclear weapons for communist unification called as "Juche Revolution" in a lot of speeches.
As North Korea's economic isolation and the crisis of system become worse, the obsession with the nuclear development and communist unification became more serious. In the 1990's massacres, dictator Kim Jong-il spurred the speed of development in WMD(weapons of mass destruction) and had completed nuclear weapons as a matter of fact. This is because the North Korean leaderships believe that nuclear weapons and communist unification can be their only achievement in the history and is the only way to prevent the collapse of the regime.
In the end, there is one common way to stop the North Korean nuclear problem and the horrible communist unification. It is the liberation of North Korean people and the unification of liberal democracy.