The dragons were actually peaceful beings. They could come in any size, they rarely made an appearance, and when they did, they seemed like ghosts, moving quickly and disappearing before you even knew you saw one. They wouldn't stay in once place long.
All that changed when, thousands of years ago, a prince decided he wanted one of the great beasts as his own, to tame it, perhaps even raise it. The prince managed, after years of nonstop searching, to find a nest where a mother dragon was keeping her small brood of eggs. He stole one when she left to get food, but as he as climbing down from the nest with the stolen egg, the mother caught him.
Angry, but still sticking to her peaceable nature, the mother tried her best to take her egg back from the prince without hurting him. When he wouldn't relent, and even went so far as to attack her, she was forced to kill him to get her egg back.
When his son didn't return as planned, the king became anxious and demanded that a search party go out looking for him. They returned with his broken body and told the king that they were sure the dragon had done it. Swearing revenge on the species, the king decided he would would enslave them. They would become little more than domestic animals.
His plan worked, to an extent. Soon, dragon-pulled carriages were a common sight, and farmers used the beasts to pull their plows. Upper class children had the smaller ones as pets. The larger, more untamable ones were pitted against each other in the arena. They fought for the prospect of getting food. The losers would either die a few hours later from a fatal wound inflicted by another dragon, or be killed for its failiure.
It was a common sight across the world now, as they had started trading dragons for this purpose. There were few wild dragons left, and when they were found, they were dragged back and forced into working for the humans.
But there was one dragon no one could catch. This dragon was undefeatable and hell bent on destroying the humans for what they had done to his once peaceable and thriving species. He was huge. Bigger than mountains, even, standing up his head would brush the clouds.
He has been destroying traders, breeders, and arenas, and setting free the captured dragons kept inside. The humans still haven't gotten a clue that this dragon, nay, this Titan, is going to eradicate them one way or another. His intent is that only dragons will roam the earth.
Whose side are you on?