The doctor had never been good at keeping his memories in the past. That's why he started running, that's why he will always keep running. Or at least try to.
Jack had never been good with running. He tried hard to keep his memories just that, memories. He's never been good with running, but now he has to run fast because the past is catching up.
There both in trouble. The doctor can't run forever he will have to stop eventually and when he does he'll be stranded in the sea of his own memories.
Jack hasn't got much of a head start and if he slows down even for a second his memories are going to wash over him like a tidal wave.
The doctor needs jack to show him how to deal with his memories, but he'll never be brave enough to admit it, he's lost enough already.
Jack needs the doctor to teach him how to run, but he'll never be brave enough to admit it, he doesn't want to lose anyone else.
