“The thing is, I thought I knew him, mum. I thought me and him were…. And then he goes and does this. I keep forgetting he’s not human.” - Rose Tyler, The Christmas Invasion
The minute the Doctor walks into Jackie’s flat, he has a realization.
Smiling at Rose Tyler in this new regeneration is different. Before, when he was born in battle and so aching and raw, his smiles at Rose had been like intentionally peeling back a piece of armor, opening it to show her what was inside. Initially difficult, more comfortable over time, but never, ever effortless.
But now, in this new body, smiling at Rose Tyler is the most natural thing in the universe. It’s as much a part of his nature as his lungs filling with air and his twin hearts beating in syncopation. His eyes smile at her, even when his face doesn’t. And the movement of his cheeks, the curl of his lips, is simply a reflection of feelings so overwhelming, he couldn’t stem them if he tried.
Mind you, he hasn’t the slightest inclination to try that at all.