[Terry576]

My friend played both sides simultaneously, until he stopped going Stormcloaks because

Spoiler

Ulfric tells you to attack that city you're the Thane of - Whiterun, I think? The people there have been nothing but nice to you, the leader was gracious enough to make you Thane, give you a house and one of the better companions in the game, and he's telling you to attack it.

Ulfric is kind of a jerk.