He tries to smile as Jo Grant leaves the TARDIS. For all the jokes about him turning into a baby and getting him into trouble with the Time Lords, her speech hurt more than he'd revealed.
He wouldn't just leave…
But he had. So many times, he'd left because it was too hard to say goodbye. He stole the TARDIS and left Gallifrey without saying a word. He'd left Susan on the other side of the TARDIS door—a decision he curses when he's alone with only his thoughts. He'd left Sarah Jane in Aberdeen, stranding her for years. He'd left Donna—wealthy and happy, true—but without the memories that made who she was. A payment for the time he'd stolen.
not forever
He looks so young that it's easy for his companions to forget how old he really is. The loans he's incurred over the centuries keep coming due, with compounded interest. Because no matter how much others would deny it, he is the inheritor of his own debts.