The Doctor stopped walking, scowling right back at the master. "I've changed? I've changed? I've grown up! What have you become?" he accused, the pain barely concealed in his voice. "The Master? Are you never going to give this crap up?" he asked, crossing his arms over his chest.
Here he was, following him back to the TARDIS like a fool. What did he think? That because the Master had come to his rescue when he'd asked? Of course, he hadn't thought that the Master was actually showing that he cared... ut a part of him couldn't help but cling onto the thought. "You're pathetic!" he yelled, quite aware that nothing would annoy the Master more. If he hadn't changed, he was going to hurt him anyway. Might as well be punished for actually doing something.