I think its only because the US has recently increased sanctions...

I think its only because the US has recently increased sanctions on it to increase pressure.
They were supposed to stop sanctions after their summit.
Also, re " denuclearisation" ....... Kim was to stop testing, and to close his nuclear manufacturing facilities.
I saw no mention of him having to surrender his existing nukes. A demand to that effect came later from Bolton.
I think he would be silly if he did. And he hasnt forgotten iraq..... I remember on tv news seeing Saddam destroying his Scuds..... and when he had nearly completed that, we attacked, destroyed his country, and hanged him. Same with Gadaffi. Kim himself has referred to that.

I think a lot in both Koreas like the idea of reunification...after all, it was only the US that caused their separation... but the US wouldnt allow that..... it would have to shift its bases and troops, essential to their encirclement of Russia ....and China.
A pity.
And, a US invasion of NK by the US would be met with force by Russia and China...... who dont want the US on their borders.
So, we'll have to see what happens.

GZ