Why should Iran be forbidden to go nuclear ?

Personally I fear for my life that more countries are nuclear capable and that this world would be better off with less nuclear. But why should USA and others are allowed to own nuclear weapons even by hundreds-fold while Iran is not ? Of course its easy to p judgement of self righteousness while one is standing on one side of the fence. Inherently no country can claim to monopolise on the question of right and wrong and that God has not bestowed Bush or USA the mandate to be the judge of last resort.