No spoilers here but a lot of ingame lore from Skyrim. If you want to figure out the entire backstory and political situation yourself, you might want to skip this post.
Trying to choose a side in Skyrim's civil war is hard, bth sides are just such assholes. The Imperials want to stop freedom of worship in Skyrim not because of any real threat but because the elves don't really like it and the actual govornment system is pretty much full of their puppets. On the other side while the Stormcloaks are in theory fighting for freedom, Ulfric's method of doing so is to murder the high king in cold blood, technically during a legal challenge but still in cold blood (I know that sounds like a spoiler but it really isn't, it's already happened by the time the game beings) then escape the city via a bribed guard (or maybe otherwise convinced, I'm not 100% sure about the guard) and sit on his throne while you kill the Imperials. He's a coward, the Imperials are fachist assholes and my characters want no part of it.
The annoying thing is, I actually agree with Ulfric's agenda, if not his methods. If he'd stop being such a coward, hiding while the fighting happens and terrifying civilians, if he'd actually strike at the standing military and loyalist Imperial enterprises and companies instead of anyone who isn't a dedicated Stormcloak, I'd happily fall in behind his little rebalion.