It is a theocracy that wants nuclear arms.
I'm puzzled how that wasn't clear from the first comment, or the article.
That's a pretty black and white way of viewing things. Hassan Rouhani is far from an extremist and has taken many actions to liberalize the country. Go look at some polls about the opinions of Iranians on the western world (and the US in particular). They are surprisingly the most sympathetic in the arab world.
But let's suppose for a moment they really do want nuclear weapons. This probably is't a good thing, but I reiterate, why must the west intervene in this? Did we intervene when Pakistan obtained weapons? Our history of interventions is full of hypocrisy and serves to make us more and more enemies around the globe while getting us nothing in return except for deep profits to weapons manufacturers. One could even argue (although I'm not sure it is really true myself) that when enemies obtain these weapons it is actually a stabilizing influence since the stakes of conflict increasing are too high for each side to escalate a conflict.
Watch Team America World Police if you want a more humorous illustration of blowback than I can do.
Explain what?