Fact is that the Iranians are the most America-loving people in the Middle-East and that is only the governments who have problems with each other.
Say what? I doubt you know much, if anything about Iranians and their history. Quick recap:
1. US toppled democratic Iran's government and put in power brutal dictator, who brought a lot of suffering to Iranians, till they finally got rid of him.
2. US not only supported Hussein in his war against Iran, but also provided with chemical weapons of mass destruction, a lot of Iranians died from it. When few countries in UN proposed resolution to condemn Hussein, US veto'ed it, and said its Iran who is using chemical weapons, not Hussein.
3. US shot down civilian Iran's Air Flight 655 and killed 290 people. Not only US refused to apologize for it (remember Bush words "I'll never apologize for the United States. Ever. I don't care what the facts are"), they gave honorary medals to marines who did it.
4. US has Iran under sanctions for over 30 years, and is using all leverage it has to choke Iran's economy completely.
5. US committed so many atrocities in the region (including support to Israel in its terror against Palestinians), that along with a bitter history of US crimes against Iran itself, in the demonstrations you can often see banners "death to US", etc. Its not happening because of some whim, its a reaction to what US does. Much like you can see Jews hating Nazis, it didnt happened on a whim either.