Maybe he's just human and rationally wants himself and his family to get out alive? Anyone with a brain and a little knowledge of history can see how easily things can go badly for a dictator and his family.

If Kim Jong-Un can pull things off right, he and his familiy will be able to live out their lives without having to fear being deposed and murdered, probably keeping enough wealth to live in luxury for generations, and maybe going down in history as a well-regarded peacemaking hero on top of it all.

It's impossible to know for certain, but perhaps having invested so much in nuclear weapons and finding that it only made the US more belligerent, they realized that they had to shift their strategy? Or maybe they've been waiting for the opportune moment to extend the olive branch? Become a nuclear power so that they could negotiate from a position of power and get better terms?

It's not that complicated. To get the best concessions you come with biggest chips of your own to trade. Even if NK's intention was to negotiate from day one, they would have gone for nuclear weapons first, given that prior negotiations did not net them satisfactory concessions. What's changed is that they believe they have big enough chips now and Trump is high-variance enough that the gamble is worth it.