An old and oft-used theme (in the past decade anyway--I've only started to comprehend how much the NAs affected me while growing up), reworked into what you can think of as either a prose poem or an almost-drabble. I'll let you decide.

TRANSIENCE

"Hullo," he says, "I'm the Doctor," and he smiles. It is not easy for him, balancing between enthusiasm and politeness, treading a line between friendliness and wariness. But too soon he drops his guard and welcomes this new person into his life, throwing open his doors and sharing his adventurous life. And he knows that she will leave, as all the others have left, and he knows that eventually someone will replace her, as she has replaced someone else. But still, he smiles and says his name, and hopes that this time she will stay long enough that he can forget for a while that someday he will have to introduce himself to someone new.