Thorin's ancestors (Thrain the Old) mined in the mountain, and got very rich working for Men and Elves an the people of Dale, their fathers begged the dwarves to apprentice their son's, and Thorin's grandfather was king. Dragons in the east were stealing gold, riches, and jewels, and killed anybody in their way, until gold became scarce in those parts. All the gold in the mountain was probably what brought Smaug, a particularly more dangerous and more deadly dragon, to their mountain. All they heard at first was a sound like a hurricane, and the dragon came, a ball of fire. It killed all the dwarves that came to fight, and the rest of them in the mountain. It stole all of their riches and gold, and when he had finished with the mountain, he destroyed the village of Dale, leaving it in ruins. Very few dwarves survived, some being Thorin, and his father and grandfather, and they had to make a living as blacksmiths.