One banner which reappears time and again in the recent election protests: "Coup 1953, Coup 2009". Iranians (who like other middle easterners appear to have longer attention spans than we westerners) haven't forgotten how 56 years ago, their parliamentary democracy was violently overthrown in CIA led coup, Operation TP-AJAX, to be replaced by a vicious dictatorship, in order to further the profit margins of private US and European (oil) corporations.
Obama even acknowledged this event in a recent speech in Cairo to the world's muslim community where he maintained that the US "isn't at war with the world's Islamic community" ... honestly... (!!). But then he admonished Iran regarding "behaving themselves in the global community", as regards their alleged (but unproven) program to manufacture nuclear weapons. Such a flagrantly duplicitous statement doesn't go down well with Iranians, makes the US appear even more arrogant and is more of a diplomatic faux pas than helpful. The US (and Iranian) people would obviously prefer stable relations between the two countries, but as far as the US (and current Iranian powers-that-be) are concerned, that seems to be entirely a different matter.
Its probably more about flowing testosterone on both sides, and the paranoia of "losing face", or seen to be "weak".... not to mention the recurring problem of hardline religious conservatives such as Iranian president Ahmedinejad, and others who shall remain nameless