Disclaimer: I don't own Doctor Who.
It's always good to have a Smith on board.
Smiths are useful. Where Tylers are unusual and Joneses are love struck and Nobles are batshit crazy, the lot of them, the Doctor thinks maybe Smiths are even better at helping people than Doctors are.
First, there was Sarah Jane. The Doctor isn't exactly positive he knows anything at all about Sarah Jane, because she just is. Sarah Jane is not raucous or disorderly, nor is she diffident and neat. But Sarah Jane is clever - oh, yes, Sarah Jane Smith is clever.
Then comes Mickey. Mickey is the antithesis of Sarah Jane - disagreeable at the best of times, and not pleasantly so. But the Doctor notices that Mickey is useful. Mickey is clever and bold - Sarah Jane and more. Mickey is human and proud of it, sound and steady in his normality. Mickey is a person, and a boy, and Mickey Smith is good.
The Doctor has never formally met Luke, but he's a Smith, too, and that tells the Doctor far more than hours of companionship and chitchat could. In the short glimpse of Luke the Doctor got, he saw Sarah Jane's heart in the body of a young boy. And Luke has Mickey's hands - hands you can tell, from just one look, are made to help. Luke is Sarah Jane plus Mickey and then some, but mostly, Luke Smith is new.
The Doctor likes to call himself John Smith, for when he can't go by Doctor who? Just Doctor. He likes the anonymity, the enigma that comes with being a John, because really, no on remembers a John. And he likes to be a Smith, because Smiths are bloody spectacular. They've taken their stunted, human selves, and clawed their way to the top, and in the end, Smiths always stand victorious. Their hands may be bloodied, they may be exhausted, but Smiths win.
The name Smith is like a lucky pair of socks - mismatched, worn out, but every time you've got them, you can't help but feel a little special.