You're not going to like the answer, but here it is. There are too many humans on this planet. We are using up too much resources.
I think this is accurate. I saw a lengthy interview in the NY Times with Kim Jong Il about six years ago. He was really quite practical. He said something along the lines of, "Of course, I know that the US would destroy me in short order if I ever really tried anything significant. But I also know that if I did not build nukes, the US would never talk to me." Like NK, most countries are ruled by narcissistic megalomaniacs. It is all about them, and the worst thing they can imagine is to be ignored. The people of North Korea are doomed to generations of subsistence and repression, and possibly much worse. It's a terrible crime against humanity that probably no one knows the true extent of, and it has been going on for longer than I have been alive. I hope to see an end to it one day.