Midgardsormr's condescending attitude comes from how he feels you're an insignificant speck next to him and common draconic arrogance. At that point (Before the Fall - 2.55) he doesn't really feel you're worthy of being Hydaelyn's chosen; it's not until you reach the endgame of Heavensward (3.0) that he understands why she chose you.

Just as well dragons are known for their extraordinary arrogance in fiction; it's one of their defining traits. They always know better, are stronger, smarter, more experienced, older, etc. Midgardsormr, King of the Wyrms, was bound to have at least some arrogance in his character pretty much from the get-go. Even good dragons are still arrogant. (Ironically, the only major draconic character to buck this trend is Tiamat, who is usually evil and accordingly characterized by her grand arrogance; here she's the most humble of them all.)