Noam Chomsky spoke at Harvard Memorial Church on March 6. His analysis, largely supported by the public record, is that the constant threat of force by the U.S. is driving the Iranians to develop a nuclear weapons deterrent. Paraphrasing, "They'd be crazy not to."
There is very good historical parallels with North Korea to support Chomsky's analysis. The U.S. "talked tough" on N. Korea and marginalized its leadership, keeping "all options on the table" through the last 60 years of administrations. They offered unilateral talks continually to address the threat the U.S. posed to their sovereignty, all summarily denied. They offered to sign non-proliferation treaties, which the U.S. ignored.
Now they have a burgeoning weapons program.
Contrast the situation with Iraq who had no substantive nuclear weapons program at any point in their history. The U.S. has decimated that country with violence.
What's the lesson?