All-Seeing I wrote:
The way I took it: he'd regenerated recently, just time enough to recover from its effects and get back on the trail of the Nestenes. The photos that Clive showed Rose - the Doctor did a bit of travelling between leaving Rose with Mickey, and then deciding to go back and dangle the time-travel carrot in front of her.
Yep, perfect opportunity, right there. And those solo travels may have convinced him again that he needed a companion and to try again with Rose.
(After a stop off to play Father Christmas when she was 12, perhaps, haha.)