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A new face brings new issues with it, the biggest one being convincing people that you're still the same person, but not. There is always the problem of running into old friends only to find they now look older than you, which can be particularly embarrassing with humans, as they do get so very sensitive about their age. It doesn't really bother him, other than to remind him painfully how very short their lives are.
He's in the middle of the 48th century, trying to stop a dastardly invasion by some shady and as yet unidentified aliens, when he runs into another time-traveller and her glamorous assistant. He's a bit distracted by missing having one of those of his own, so its not until they've just made a daring escape and are mid-planning the equally daring rescue of said glamorous assistant (who insists on being called Bob, despite a predilection for wearing sparkly cat-suits) that she grabs him by the wrist and the familiar double heartbeat stops them both dead. A quick touch of telepathy later and they're hugging madly and demanding explanations of each other at a hundred miles an hour. Naturally this gets them recaptured rather promptly, however it gives them a chance to catch up, and Bob's a resourceful girl, she'll get them out eventually.
It takes Susan approximately twenty minutes to get past the initial shock of not being alone in the universe and start winding him up. The Doctor does the best not to pout, mainly because he feels pouting is something his last self would do, and the cosmic angst thing seems a bit last lifetime. Also because he's quite sure pouting will only make her worse. Discretion being the better part of valour at this juncture, he lets her enjoy her moment of triumph. She's earned it after all, spending as long as she did travelling the universe looking to the rest of the universe like a teenager when she was closer to 100 (still a teenager by their standards, just a bit galling being talked down to by humans) had to grate after a while. However he retains the right to sulk a little that she's clearly never going to call him grandfather again, it may have been a daft earth ritual that they'd picked up to help blend in, but he'd liked it dammit. She's his granddaughter, and half his age, even if she looks twice his to human eyes, having her call him Doctor just isn't the same. She calls him his true name then, so he calls her by hers and somehow everything slides into place. They know who they are, in themselves and to each other; everything else is just semantics. The rest of the universe can think what they like, they're the last of their kind, and they'll make their own rules. With his arm round her shoulders and her head fallen on his, there's nowhere in the universe they'd rather be.
They try explaining it all to Bob when she turns up to rescue them but she just mutters about aliens, paradoxes and 'can we just escape already'? So they do.