"Can I talk to you?"
Rose Tyler looked up. The Doctor-who-wasn't-the-Doctor was standing in front of her, looking uncharacteristically awkward.
"Yeah, of course," she said, sitting up. "What about?"
"It's just- well, I know you're disappointed to be left here with me-"
"I'm not disappointed." Rose spoke over him, pausing a long moment to think. "It's like- it's like when you're a kid and what you want for Christmas is a Barbie doll. That's all you've been begging for for weeks, and you haven't got any dolls and you really, really want one."
"Sorry, but is this- is this relevant?" the Doctor asked suddenly.
"Yes, very."
"Oh, okay." He sat down across from her, looking serious.
"Like I was saying, you want a Barbie so badly, and your parents have heard about it so many times they're ready to kill you. So on Christmas morning, you get a present, and it's a doll, and you're really excited, but then you realize- it's not a Barbie. It's pretty the exact same thing, same look and all that; it's just not a Barbie.
"And as you're looking at it, really confused, your mum's going on about how they were going to get you a Barbie, but they were so much more expensive, and this is really the same thing. And now you're feeling guilty about wishing for a Barbie and you open it and look at the doll and, 'cause you're a kid and you sort of believe this kind of stuff, you're thinking how the doll must feel, 'cause it's not her fault she's not a Barbie. And you're thinking how glad you are to get your first doll and you're definitely happy about it, but you can't help thinking that you wish it was a Barbie, even though you know months later you'll adore the doll and not care what brand it is.
"That's how I feel about you." Rose stopped for a second, looking at the Doctor, considering. "I'm definitely happy to have you here with me, but I can't help wishing I was with the other you, travelling all around the universe. At the same time, I know that years from now, I won't care about which of you is there, I'll just be glad to have you with me."
The Doctor was looking at her, leaning back in his chair, arms crossed, one eyebrow arching skyward. "So, you think of me as a Barbie knock-off?"