The Voyage of the Dawn Treader
He had turned into a dragon while he was asleep. Sleeping on a dragon's hoard with greedy, dragonish thoughts in his heart, he had become a dragon himself. (6.32)
In spite of the pain, his first feeling was one of relief. There was nothing to be afraid of any more. He was a terror himself now and nothing in the world but a knight (and not all of those) would dare to attack him. He could get even with Caspian and Edmund now....
But the moment he thought this he realised that he didn't want to. He wanted to be friends. He wanted to get back among humans and talk and laugh and share things. He realised that he was a monster cut off from the whole human race. An appalling loneliness came over him. He began to see the others had not really been fiends at all. He began to wonder if he himself had always been such a nice person as he had always supposed. (6.34-35)
It was, however, clear to everyone that Eustace's character had been rather improved by becoming a dragon. He was anxious to help. (7.14)