Because the Dragons wish to prove themselves to their father Aka, and defeating his avatar would do that.BathorysGraveland said:If all that is true, then why do they even bother to fight you? Sure, there would be the brave few who would fight to the death with courage, but the majority? Why not surrender if they have no hope? All that just makes the dragon seem like a minor enemy to you, not some proud warrior-race to fear, just something extra to kill.
On top of that, beating you would make them the strongest Thu'um, and thus have the right to challenge Parth for leadership.
Dragons are prideful, and seek power, but if you show them the Thu'um that beat Alduin Epoch-Eater, they will submit.
Also, both Odahviing, and Durnehviir, show that if you kick their ass, they will submit to you.