Why are dragons always slain? It's stupid and repeated, and I see it everywhere. very annoying?

Humans are cowardly creatures, in my view humans are naturally prey but they have weapons...they kill all that scare them. While we dragons are peaceful humans don;t like that we can breath fire...or some pathetic stuff like that. They like our barf too for some reason. The only dragon slaying that I admit was somewhat okay was dragonheart...sure sad but it was part of the story..