Kind of obvious reallly its because your the dragonborn!

Slayer or Alduin veteran of battle feared by all in the battlefield you are the ultimate badass who slays dragons for a job, and explores dungeons for fun. If the Thalmor heard about you (which they did of course) your average Thalmor would [censored] his pants at the thought of battling you, and your nord/imperial allies would have a huge morale boost. Itd be like this.

Thalmor soldier: I cant wait to battle those ridicoulous savage nords in Skyrim we will show them who the true Gods are!

Thalmor Magi: Haha friend the dragonborn will be there too I will incinerate him, and all of his allies!

Thalmor soldier: DUDE THE [censored] DRAGONBORN IS THERE [censored] THIS IM OUT.

Thalmor magi: Don't be such a coward fool, the dominoin will have your head!

Thalmor soldier: WHATEVER BRO IM GETTING THE [censored] OUT THAT DUDE EXPLORES DUNGEONS FOR A LIVING, AND KILLS DRAGONS FOR FUN IM OUT.

Thalmor magi: Fool the dominoin will prevail no matter who fights against the- oh look a foolish nord come to challenge the thalmor army. I will incenerate him!!!!! Odd a storm just started when that nord came OH [censored] ITS THE DRAGONBORN