I believe the reason for that is because of how unpredictable NK is from the point of view of other states. This lack of predictability creates insecurities among other states, especially SK. Thus if they can't make NK predictable, they will try to reduce the potential for NK to become somewhat more threatening.
However, from the point of view of NK, it is quite isolated from the world in terms of diplomatic relations. Because of this and the relatively small size of the country, it is probable that they want seek some sort of method to deter other countries from acting aggresively against it.
So in short, both sides just want to become less insecure about potential threats from outside their state.
Btw, is this in the wrong section?
EDIT: nvm this thread has been moved to the correct section