ITT we come up with reasons for each race to hate the Empire and side with the Stormcloaks
Imperials: renegades who know about inner corruption in the Empire, who is trying to silence them
Nords: well, nationalists who believe in the Stormcloaks cause
High elves: Thalmor agents setting out to weaken the empire by aiding the Stormcloaks
Can you think if any for the rest?
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Lore wise, Ulfric Stormcloack is just a puppet of the Thalmor which is the real enemy. I mean, the Imperials are puppets of the Thalmor also. How about neither.