To the outside world almost everything that North Korea (Democratic People's Republic of Korea) seems insane. Even with a million man army, and thousands of tubes of artillery they could not possibly hope to retake the South (Republic of Korea). Even if they did there is no way they could possibly control the ROK. The only support if you could call it that, that the DPRK gets is from China, which often uses DPRK to keep the West focused elsewhere. As useful idiots go DPRK is pretty useful to increase China's regional power. The problem is that China fears a united Korea, they fear a Nuclear Japan, and they fear the Republic of China in Taiwan. Everything that DPRK does has to be carefully watched and controlled or PRC's worst fears might be realized.
It doesn't help that the animosities are not just cultural. East Asia is one of the most quietly racist areas of the world. Behind the veneer of polite manners deep seated hatreds lie in centuries of history and plain societal distaste. The Chinese and Vietnamese hate each other, the Koreans and Japanese have a long history of hating each other, most of East Asia still holds resentment against Japan for the "Greater East Asia Co-Prosperity Sphere". Mentioning the Rape of Nanking or the horrors of Manchuko would be beating a dead horse. Sadly because Asian societies are somewhat circumspect, most Americans do not pick up on this deep seated hatred. When DPRK wanted to negotiate directly with America and we demanded 6 way talks with everyone in the region, we were doomed to fail diplomatically precisely for the reason that these societies, though seemingly similar all quietly loathe each other.
We also can not know what over 60 years of isolation and the warping of the DPRK citizen's minds by one of the most intricate propaganda machines in the world might do to the people or the country. Do they really believe what they say about the Dear Leader, or do they only say it because they have to? We don't know. It's perhaps the greatest coup in counter intelligence history. It's the first time in recorded history where we simply have no ideas what the intentions, personality or the wishes of a leader of a nation are. Is Kim Jong Un playing the brash saber rattle to strengthen his pull with the military? Is there some actual endgame here? Is this to get concessions? No one actually knows. Perhaps a more terrifying possibility, is that he's so surrounded by sycophants that he might actually believe the things he's saying. That his forces are nigh invincible.
If there is one lesson War in the latter part of the 20th century taught us, it is that Mass (large numbers of troops attacking without real leadership) can beat Economy of Force (well led troops marshaling their force to strike in strength at precise points). The suicidal wave attacks of Stalingrad, Chosin, and several battles in Vietnam, Iraq and Afghanistan have proved. Western doctrine is to use air power, and speed to our advantage, but when we have neither our only choice is to fight as hard as possible, this doesn't always work.
Regardless of how the war is fought casualties will be appallingly high. If DPRK attacked you could see hundreds of military casualties and perhaps tens of thousands of civilian casualties in the first couple of hours. All the major bases the US has along the DMZ are already targeted, and there would be probably no warning of an actual attack. There would be artillery for sure. More than that is questionable. DPRK's air force is antiquated to say the least, there might be tank columns even if said tanks are antiquated. That is if they can get through the million plus mine DMZ. Ultimately there is no way that they could win. Even if they could somehow take the entire Korean peninsula, they could never hold it. The population of ROK is too well fed, enjoys their freedoms too much, and there are too many of them. Similar reasons exist for why the ROK has no desire to go North. It would be like the West Germany/East Germany unification, only worse. It would take decades to rebuild the DPRK and actually educate (not RE Educate) the population.
For all these reasonable and practical reasons not to go to war, would they? Well we're seeing things from the outside. Things that seem sane and rational to us, may not be so to the DPRK. We literally have no idea what drives that nation. What social pressures are there? What are their needs? How much of the culture of celebrity surrounding the Kim dynasty for real and how much is for show? What can their rockets really do? The biggest problem is the same problem we have with everything to do with DPRK; WE DO NOT KNOW.