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?