The US secretary of defense [James Mattis] says he “cannot imagine” the US ever accepting North Korea as a nuclear state, and claimed that possessing nuclear arms makes Pyongyang vulnerable rather than more secure. (Source)

Why do they think this?

They think this because US imperialism simply cannot accept that the DPRK has the ability to resist their global interests. Since the end of the Korean War, the US has embarked on a campaign to destroy the country politically and economically. The DPRK remains one of the few bastions of anti-imperialism in the world. It is unacceptable to the imperialists that anyone should make themselves capable of countering the threats they make against oppressed countries.

This nuclear weapon development by the DPRK signifies a significant new stage in the evolution of their antagonism with the US. For decades the DPRK has had to make terrible sacrifices in order to maintain their independence. A lot of resources went into military production to stave off a likely military invasion. The massed artillery along the DMZ has always served as a deterrent against invasion. These nuclear weapons under development serve the same purpose.

Since the US has always harmed the DPRK and threatened them with invasion (and now nuclear annihilation under current President Donald Trump) the country began producing nuclear arms capable of reaching the US. The risk of invading the DPRK has been made much more deadly than it has ever been in the past. The desire of the DPRK is to be left alone to engage in economic activity as everyone else does – to engage in trade as other countries are allowed to.

By no means, does possessing nuclear arms make the DPRK more vulnerable, quite the opposite in fact. The ability to soon strike the US mainland with the most powerful weapons ever known forces the US to think twice about invading the country. Secretary of Defense Mattis and US imperialism “cannot imagine” the DPRK as a nuclear state, because it undermines their ability to threaten and coerce the country.

The US desperately wants to block the DPRK from a nuclear capability because it undermines their power over them. The DPRK will not submit to the will of the US and its desire to conquer them. Such a thought is a product of Mattis’ imagination.