Rose isn’t quite sure exactly when it was that she fell head over heels in love with the Doctor. She thinks it was most likely after his regeneration, because although she’d loved that brusque, big-eared man in the leather jacket, something about him had held her back. Perhaps because he looked around her mother’s age, whereas he seemed closer to her own age after he changed.
Maybe the when doesn’t really matter, because it happened, and now she can’t imagine not having him in her life. She doesn’t care that he’s not human; he’s still the only man for her.