Officially, North Korea wants nuclear weapons based on their philosophy of Juche or "self-reliance". Given the collapse of communism and being utterly surrounded by various "capitalist" countries, they remain deeply paranoid about being invaded by the outside world. What it comes down to is they don't want to be dependent on anyone else for their own defence (even an ally such as China).
Whether you believe them is up to you, but the greater danger with North Korea isn't necessarily them attacking the South (as they would prefer to unify the whole country) but that the North Korean regime collapses and the nukes are either used in a civil war or "disappear" and get sold to someone and end up in the wrong hands.
