American was at war with Japan and Germany not to long before as well. Now we are best buddies. Things can change if the money's right.
Neither of those solutions was 'all good' so at some point the people will say 'enough'. It can take a while.
The purpose of the Guardian Council [0] is precisely to prevent such a turnaround.
All countries (or other groups) can find reasons to hate or love each other. Look at France and Germany, close allies now but plenty of opportunity for hate. All it takes is an effective political leader to manipulate the public in whatever way suits them.
The beginning of Mohammad Reza's reign was in 1941 and had nothing to do with the US. The coup (by both the US and UK) was about reversing the nationalization of Iran's oil industry, not about installing Mohammad Reza as king. Britain and the Soviet Union did that in WWII.
Iran held US personnel as hostages for a year and imprisons our visitors. They are also sworn enemies to US Allies like Israel.
Iran is "bad" because they are our enemy. And they see the US as "bad" because we've been their enemy in the past.
There is absolutely nothing strange about the hate-filled relationship between Iran and the US. It is simple politics and history.