User blog comment:Twistedsystem004/Imperial Or Stormcloak? (Emma's Thoughts)/@comment-10207438-20130923210428

He fights for the people of skyrim, who are systematically being murdered because they're seen as threats by the Thalmor.

The Thalmor do not hide the fact that as soon as they gain their strength they will fight against the Empire, yet the Empire forces it's people to bow down to the Thalmor.

By becoming High-King Ulfric can remove the empire's control on them and force the Thalmor out, if they stay they will start the next war much earlier than they can, and they have no way to keep the people of skyrim controlled. If they start the war they will have no way to reach Skyrim without plowing through Cyrodiil first, which of course the Empire wont allow; The best interest in the next Great War would be to start it as soon as they can, BEFORE The AD is sufficiently prepared to destroy the Empire and all human territories.