"Got bored of the old body," he lies, pretty obviously. He needs to be careful about revealing information about his past. Sure, he more or less trusts himself not to use sensitive information, but he doesn't trust the Master.

Now he's reached him, he's suddenly not sure what to say. So he goes with scolding him. "Listen, under no circumstances are you allowed to harm anyone here, you got me?" he says sternly, pointing a finger at him.