He had to.
He had to pick her up, had to save her time and time again, had to clasp her hand with his every chance he got. He just-
She's the most important person in all of spacetime. To him, at least, and though the salvation of the Destroyer of Worlds shouldn't matter, least of all to the Destroyer himself, he finds, eventually, it does. She makes it matter, makes him matter, when he thought nothing ever could again.
And he hates to say it, he really does, but-
She makes the Time War worth it. All that death, all that destruction, all to end a war that would have obliterated her.
He would do it again in a heartbeat, if only to save this one ape. No, not ape. Human. This one bright, sunshine-and-lipstick, tongue-in-teeth human girl.
And he is-was, he means, he swears he does, because even though he'll be there it won't be him being there-just a old, old man in a ratty leather jacket whirling through planets and ages.
He wonders who he'll be next.
Whoever takes his place, he knows he'll treat Rose Tyler with the utmost love and respect deserving of such a wonderful Wolf, knows he'd kill himself again in a heartbeat to save her. He can feel it.
He can only hope that whoever he turns into is someone Rose can love in return.