What is North Korea's motivation? Given the UN's predicable past responses, North Korea must gain some political benefit from these UN responses.
Look at UAE and Singapore, it takes little time to change the economic situation. A little years here and there, with China's and India's emergence on the world scene things can change pretty rapidly. To cut US's influence in Asia, North Korea is a indispensable asset.
Allies are not permanent and US's help to Japan and South Korea is not infinite. Geopolitical situations will always change and have always changed throughout history.
Given this situation, North Korea can be a big player in the future, now imagine having the military might in this situation. It can force unification of the Korean peninsula on its terms in the future.
They are doing a pretty good job of being poor already. I don't think you can compare North Korea to those other countries, even China. North Korea is a world peculiarity; it is the way it is because of it's bizarre regime, in a way different from China. It is so totally isolated and the average person is so uninformed about what is happening less than 100 miles from it's borders. The kind of trade that would reduce North Korea's poverty, I think would topple the regime, a regime based on magical stories about unicorns and people that don't need to use the bathroom.
So the NK government has the choice between pushing nuclear technology and missiles, which kind of works in the short run, or they can throw all their previous work in this areas away to start a rather complicated transition to a more long term strategy. Depending on how stable the regime is, it may survive the short term problems such a change of strategy creates.
Currently they have been using the threat of attacks as a bargaining chip to extract things like food shipments from the US and Japan.
PRNK's behavior right now seems to be about pushing the USA back to something like the prior agreement, likely because they see the Obama administration as more open to this possibility. Honestly this would probably be a win for everyone, particularly if it included opening up a labor trade between north and south Korea. North Korea is impoverished, but their citizenry is actually quite well educated (if indoctrinated). If economic sanctions were weakened, there's little question that access to north Korean labor would be in high demand globally. There are a lot of challenges to making this happen. Possibly the hardest are social, and will simply take the passing of generations to ease.
There are also probably important PRC PRNK relationship dimensions to all this as well, of which I'm ignorant.