Korea is a very small peninsula in East Asia located between China and Japan. Following the Allied Victory at the end of WWII, Korea was suddenly liberated from the rule of colonial Japan.
Premise 1: The US and the Soviet Union then occupy the Korean peninsula and decide to share the region and bisect it at the 38th parallel (Cairo Conference). The Korean people, who share a common heritage, language, and culture had no voice in the divide of their country, which resulted in families members being separated and the atrocities now known to have been ongoing in the North. If the Korean people were allowed to organize and determine their own destiny they would be unified today. However, because of the ideological and geopolitical interests in the region, the US and Soviet Union swept aside Korean sovereignty in the name of their own interests. It is an injustice that was never corrected and the Koreans remain one of the few divided nations on earth.
Premise 2. Much time has passed since the end of WWII and the Korean War and the two countries have gone their separate ways. However, both countries have constitutions that say reunification is the ultimate goal. The political interests of both counties from the start was to get back together.
Premise 3: Defector interviews demonstrate that North Koreans want reunification. South Koreans also agree, but young people don't want to pay for it. This is very reasonable. From their point of view, it would be a great burden to any young generation if reunification was to happen during their time. However, if we take the long view, the economic pros outweigh the cons: manpower, natural resources... The list goes on...
Premise 4: This is the weakest premise, but I'll just throw it into the mix. The 21st century has seen a lot of talk about terrorism and nuclear weapons. We know North Korea supports both. They are also among the largest elicit drug producers in the world. They recently have been charged by the UN for crimes against humanity. The state operates under a dictatorial slave state (see Song-bun for more information) of create suffering, torture, gulags, and death. They have actively threatened neighbors and looking for ways to have the technological power to send ballistic missiles to the US mainland. The international community has organized balancing coalitions against such rogue nations before, even toppled them, but because of the involvement of China's funding, the North is allowed to persist and is given carte blanche to keep doing what they are doing. North Korea is a UN member state and should be held accountable. For that reason alone more should be done, yet we don't. Thanks China.