Smaug the Golden really wasn't a terrible dragon, once you got over the fact that he slept nearly sixteen hours a day.
Thorin Oakenshield could barely recall the day the dragon had come. He knew he was there, of course- Dis often described the great creature swooping out of the sky, bellowing, lighting the air with white hot heat and flame. It was her absolute favorite part of their history lessons, and she'd been barely a twinkle of a thing when it had happened. There had been an uproar, of course. The warriors had been mustered, the children had been secured. Erebor was prepared for the worst and so was Dale.
Then Smaug had done the strangest thing.
He'd landed, quite politely, before the doors of the dwarf city, and he'd howled.
This of course was the important part, because while it was not a part Thorin remembered, it was a part that Dis insisted had changed the course of history for all of their kind. For held securely in his mother's arms, little Prince Thorin, son of Thrain, son of Thror King under the Mountain, had reached up, past the great stone door towards the crouching, howling beast, and said, "Hurt!"
Indeed, Smaug was hurt. An ill thrown javelin, later determined to be of orcish make, had been lodged in his breast, not near enough to his heart to kill him but beneath his skin and scales. Dragon scales are powerful and thick and the weapon's weight alone was not enough to drag it free; Smaug's claws could not grip it, and so he was in constant pain. Dwarvish tools removed the javelin easily enough, once someone dusted off a dwarvish-to-dragon scroll and got up the courage to approach.
For the favor of treating his injury, the origins of which he would not reveal, Smaug the Golden made a promise to Thror, King under the Mountain.
"I will safeguard your gold and your folk, and no harm shall come to Erebor whilst I sleep below."
Thror, of course, could not say no, for one doesn't say no to a reptile with teeth like swords and claws as big as scythes.
So Smaug the Golden became Smaug under the Mountain. Tunnels were widened, mines were moved, gold was piled high and Smaug slept and guarded and occasionally got himself into riddle battles with the various guards and accountants and others who would come to move some gold or bring some in.
Smaug's other pastime- and admittedly his favorite hobby- was matchmaking. Of course being so large, and sleeping so often, meant that Smaug did not have time or energy to do the running around required of a draconian matchmaker.
So he determined to find himself an assistant.
That is where our story begins.