As some of you stated, the south and the north are seperated because one is communist and one is democratic. Japan and China were both looking to take over the entire Korean penisula and when China did invade the US pushed them northward.
In my opinion it would be bad if North and South were to unite at this time.. at this point in time we KNOW who is the foe and who is the bystander, if we mix among them then more innocent people is at risk of being killed.
North Korea is still using spys to smuggle South Koreans and Japanese people and using them to train the Army on learning their language, lifestyle, so that they can have spys in other lands without them sticking out like a sore thumb.
There still is cases of North Korean residents fleeing to South Korea because of the cruelty of the communistic regime.. they literally starve their population so that they can build a stronger army. Peace treaty will be no good unless they covert to democratic system first or else the US will have a bigger problem on their hands.