People who hate Iran and see it as America’s enemy do not trust Iran to negotiate in good faith over its nuclear program. Some of these people are simply xenophobic. Others in the non-trusting category have simply paid close enough attention to Iran’s previous behavior to be skeptical of its trustworthiness. Here is why they might be right.
Iran, like other countries in the Middle East, is a prototypical honor culture. That means that above almost anything else, the people of Iran value reputation, and they tend to make the defense of reputation the center of social life. Men in an honor culture want to maintain and defend, by whatever means necessary, a reputation for toughness, strength, and bravery. Women in an honor culture want to maintain and defend reputations for chastity and loyalty. Loyalty is less central to a man’s reputation in an honor culture, but only because strength, toughness, and bravery are particularly important. Loyalty is still important to men. But loyalty must have a social reference. Loyal to whom?
In an honor culture, that social reference is the honor circle, which begins with family but extends to the community and even the nation. In honor cultures that are especially collectivistic (which the most extreme honor cultures tend to be), the communal nature of honor is particularly important, so the honor or dishonor of the community becomes the honor or dishonor of the individual.
Because of this dynamic, those outside the honor circle become the enemy, against whom the respect and value of the ingroup (family, community, nation) are evaluated. Victories over the enemy become the sources of honor for the ingroup, like the Native American tradition in war of "counting coup" against an opposing tribe. Thus, lying to members of the outgroup is not truly considered a moral failing. It’s not even a true lie. It’s a trick, one-upmanship, counting coup. To “pull one over” on the enemy is actually a source of honor, both for the individual and for the collective.
This is why Iran cannot be trusted to negotiate in good faith over its nuclear program. Its diplomats can sit in front of America's at the bargaining table and make promises and claims, but at the end of the day, their goal is to pull one over on the American diplomats. They will say whatever they need to say to get the deal they want to get, and they won’t lose a moment's sleep over having lied through their teeth at the negotiating table. They won’t feel that they have lied, in fact. The honor of beating their enemy trumps all other moral imperatives.