As a rule of thumb, I don't want more autocracies to have access to nuclear weapons. The United States has long learnt the lesson with north Korea, and they are not willing to commit the same mistake the did back when they decided not to push further after the Korean war. They have proven themselves incapable of attacking and overthrowing a regime which has access to nuclear weapons, because of the military doctrine of mutual nuclear destruction.
North Korea is a small insignificant country with little resources and still the USA does not dare to mess with them because those nukes, it is what the regime of Tehran wants to accomplish and what the United States seeks to prevent at all cost, because Iran sure is not as small and insignicant as NK is. It has vast resources and are capable of actually becoming a threat to western values.