It is very important to pay attention to what we can call it Historical Context.
As you know, Islam has more than one billion followers all over the world, but they are different people with different cultures.
Especially in making of Shiism, historical context is very important. Although it is absolutely wrong that Iranians shaped Shiism, but it is very impressed by Iranian’s national culture.
For instance, Shiism is stricter than Sunnism in politics and even Islamic laws (Feqh), but Shiism followers are more tolerant towards the other people.
I believe that tolerance in Shiism, mostly originated from Iranian culture, especially ancient culture.