I think Shia'ism is actually a kind of rebellion against the Arab imperialism brought by the spread of Islam.
Islam is basically a tool used to expand the Arab imperialism. The Persians are a very proud people with a rich history and culture. They look down upon the Arabs as a very inferior people with a barbaric desert culture. But, unfortunately, the Persians get defeated by the combined Arabs under the Islamic flag and have no choice, but to accept the Arabic imperialism. So, what they do is, accept Islam, but modify it to suit their own purpose that they don't have to be slaves of the Arab imperialism. This is like showing the middle finger to the Arabs, which the Arabs don't like to this day.