The case of North Korea, for example, shows why not all countries should have nuclear weapons. Irrational leaders that threaten the people of their own nations and the nations around them should not have weapons with the capacity to destroy millions of lives at the touch of a button. Developed nations have utilized restraint in using nuclear weapons, because using them could create a life-threatening situation for the planet; however, unstable leaders may not have this restraint.
It is very idealistic to suggest that nuclear weapons should be restricted. Unfortunately, it is very difficult to police the development of nuclear weapons in those countries determined to have them. Even if U.N. sanctions were put into place to penalize countries that violated policies preventing them from possessing nuclear weapons, the costs of enforcing them would be prohibitive.
Nuclear weapons are not necessarily the problem. The problem is in the motive of the government officials for having them. In an ideal world, nuclear weapons would not be needed. In today's world, such weapons have brought swift actions at the cost of innocent lives. Since one country is not the police of the world, nor should it be, countries have the right to defend themselves.
The problem with this argument is that it presumes that there will be a body of nations, presumably who already have nuclear weapons (US, Russia, China, etc) that have the final say about who has the authority to possess nuclear weapons. These countries obviously have their own political agendas that would be well served by having the final say about who can possess nuclear weapons. No country should have that kind of authority over another.