Why do Americans have nuclear weapons? To protect yourselves and to intimidate others...
Some people seem to forget that where foreign policy is concerned, the primary US goal has not always been human rights or democracy, but US interests. This sometimes means oil, sometimes bananas, sometimes casinos and sometimes it means stifling political movements that might go against US interests even if it meant stomping on democracy and the people's will. The US has assassinated foreign leaders and supported brutal, murderous dictators ( a goodly list of these ).
In the case of Iran the US helped install and supported a very brutal dictator in the Shah. After the revolution, the US supported Saddam's war on Iran. In recent years there has been constant rumors and threats of US invasion of Iran. Now, I think Ahmadinejad is nuts but he does not govern alone and please tell me why in hell they should trust the US? And faced with two formidable foes, Israel and the US, who repeatedly threaten attack and invasion, why the hell wouldn't Iran want nuclear weapons?
I consider the present situation in Iran to be an American creation: the US helped remove an elected Prime Minister and replaced him with the Shah and then helped maintain the Shah in power. The Islamic revolution was a reaction to this. Then the US backed Saddam Hussein's war against them. And now they should trust the US? The US should just butt the hell out of Iran and let them take care of their own problems...