Well, my Bosmer Nightblade joined the Stormcloaks because he sees Ulfric as potentionally a new Tiber Septim. He fought for the Legion in the Great War (he's a refugee from Valenwood because his family were murdered by the Thalmor) and sees Titus Mede II as a traitor. After the peace treaty he tried to assassinate the emperor but was prevented by a member of the Penitus Oculatus (ironically he's joined the Dark Brotherhood).
I don't know about a Breton, though. They have a lot to lose by a weakened empire. Maybe he believes, like my Bosmer, that Ulfric might replace the emperor? Or, if you play the main quest until you learn that you're Dragonborne you could role-play it in a way that maybe his ambitions are to take the throne for himself.
Or being a Thalmor agent and being convinced that Ulfric could weaken the Thalmor's ennemies by dividing them.
That's like a Jew joining the Nazi Party...