Why is it difficult to unify North and South Korea?

A few years ago, I heard that North Korea and South Korea will be unified but recently, people are worried of war between the two countries. I'm puzzled if the countries will unite or remain divided forever. I thought that it's easy for the two countries to unify but after reading this book about Korea by Won-bok Rhie, I learned a lot about Korea specially the reason why North and South Korea can't unify easily.

Koreans' wish would be the unification of South and North Korea. The divided countries in the world like East Germany and South Germany, North Yemen and South Yemen, North Vietnam and South Vietnam unified but Korea remains a divided country to this day. Of course no one can guess when unification will occur.

Why is it difficult to unify North and South Korea? According to Monnara book, there are three principles that are conditions precedent to the implementation of this unification plan. They are Autonomy, Peace and Democracy.

Autonomy: Since unification is a matter to be resolved by the Koreans, any decisions regarding the matter must be made by the Koreans themselves without the intervention of outside forces.
Peace: Having suffered unspeakable horrors during the Korean war, it is imperative that the two Koreas reject any attempts to resort to force and violence in the unification process so that unification may be achieved through peaceful means.
Democracy: All processes for unification must be undertaken in accordance with democratic principles and procedures.

But North and South Korea have different propositions. South Korea is Coalition and North Korea is Federal System. And South Korea can't accept the federal system plan because of the three pre conditions insisted upon by North Korea. First condition is to withdraw the U.S. forces from South Korea. Second, repeal National Security Law. Third, legalize the activities of the Communist Party in Korea.

But there are three reasons why South Korea can't accept these three preconditions. First, the withdrawal of the US military. If the US military leave the country, North Korea maintains a much larger military force than South Korea and it would break the power on the Korean peninsula. Second, the National Security Law was enacted in order to promote national security and to restrict antigovernment activities. Third, if North Korea adamantly adheres to its position to communize South Korea, the Korean peninsula could remain divided indefinitely.

The Koreans are one people in name only. They are no different from two different nations. Their ways of thinking and lifestyles and languages have drifted apart. Anyway, I hope one day the two countries will unify.