Like all Time Lords, the Doctor knows about regeneration; it’s a handy way of cheating death. You get a new body, face, and personality. Until now, the knowledge has been abstract, theoretical; it’s something he’s never experienced personally. But…
The face looking back at him from the mirror is unfamiliar, his new personality even more so. He’d never thought of the Doctor as whimsical, but the word seems to fit who he’s become. It’s like the Doctor, the one he was before, was another person altogether, someone he once knew.
Regenerating is very confusing. He’s not sure he likes it.