Negotiating with North Korea is difficult because the Stalinist communists who run the country believe they are on a sacred mission and the last communist purists left in the world. With that mentality, they feel justified in lying and deceiving opponents in order to keep the "workers paradise" going. Most South Koreans have been listening to this lunacy for over half a century and believe that the ruling group up north is slowly losing control and will eventually collapse like communist governments did in Europe. But United States negotiators point out that Asian communist parties have a tendency to adapt and hold on longer (as in places like Laos, Vietnam and China.) But the South Koreans feel they could live with a Chinese style communist government in the north. The Chinese and Japanese agree on this. However, no one is sure that the North Korean leadership feels about this. Right now, the main source of hard currency for North Korea is the export of weapons. The north gets about $15 million for each SCUD missile it ships. It is feared that the north plans to make even more money selling nuclear weapons, or weapons components. The South Koreans and Chinese are not too concerned about this, but the United States is, as some of North Koreas best customers are Islamic nations that could buy a nuke and pass it on to Islamic radicals who would use it on America.
There's another dirty little secret in the negotiations with North Korea. Most South Koreans don't really want reunification just yet. They know, from what happened in Germany, that the South Korean standard of living would take a major hit if there were reunification any time soon. The South Korean desire is for years of "transition" by the north. This means giving the north food and economic investment for as long as it takes for the north to get their economic house in order. The northerners are all for this, and want to keep producing nuclear weapons. The millions received for these weapons buys the luxuries that keep the North Korean ruling class happy.
So what it comes down to is, the South Koreans are worried about losing their prosperous lifestyle, while the US is worried about being on the receiving end of a North Korean nuke back home. South Koreans have long come to ignore North Korean threats to invade. South Korean intelligence believes that the North Korean army has turned into a paper tiger, with troops who are unreliable and likely to rebel or desert if ordered to attack the south. Morale in the north has been dropping for years and the effects of years of famine have reached into the ranks of the North Korean military. The north needs the oil shipments, and the food aid. The oil is stopped and the food aid has been cut. The north has run out of options and can be considered desperate. The south feels they can deal with this, the US isn't so sure.