Why should NK believe there is any value in negotiating? Every single nuclear deal the US has negotiated with countries which have nuclear development programs have ended up with the US/EU breaking the deal (Iran, Libya (kill the president), Iraq (invade to get rid of WMDs) and NKorea's first deal (do not provide the promised fuel oil, do not fund the promised new PWR nuclear power station)). There have been no cases where the US has honoured an agreement they made in this area.