Interesting question. There are multiple things to consider:
1. Iran has to look out for its own interests. It is surrounded by countries with nukes: India, Pakistan, Russia, and Israel. And a little further out East is China, while in the West we have France and the UK. And, of course, the world's current sole military superpower is heavily involved in two neighboring countries (that's us!!). And if you're a country that's kind of out there on its own with not much in the way of alliances, like Iran, it could be useful to pick up a nuke just to become a bigger presence on the world stage.
2. Iran isn't remotely close to having nukes. They still need a lot of work before they can start producing nukes.
3. Iran has a basket-case, third world economy. Not much you can do with that in terms of developing nukes. And even if you have nukes, you're still only marginally more threatening than before.