Maybe it goes back to the fact that he failed to save Susan from Gallifrey burning, but the Doctor has always had the urge to take care of the people that he loves. They shoot through his life like dying stars—as short lived as humans are, they may as well already be collapsing, even at birth, and he loves them, over, and over, and over again.

That's part of the reason why he's so reluctant to get involved with River. On some subconscious level, he acknowledges it—he likes to take care of the people that he loves, and River simply doesn't want or need to be taken care of—in fact, she'd probably be offended if he suggested that she needed it. River is fun to flirt with, and of course he loves her (she is a living, breathing piece of his Ponds—how could he not love that?), but he isn't in love with her, not the way that he knows that she loves him.

Then there's the fact, that, even two-hundred years since he said goodbye on a beach in Norway for the last time, his hearts still (and always will) beat for Rose Tyler. It is inevitable, he thinks, that the girl who could put the pieces of the wreckage of the Time War back together is the girl that he will always love. He wishes for her now, but he could never take her away from Nine—Nine needed her then more than he will ever need her now.

When River first turned up in the library, she placed herself in the centre of his world—made it seem like all of the things that he's done, all of the people that he's saved—none of it mattered, because he hadn't met her yet. And it made him very, very angry—the idea that anyone could waltz along and ever replace Rose Tyler in his hearts, it made him despise his future incarnation (because Ten knew that it wouldn't ever be him who got involved with someone else, even if Ten managed to last ten thousand years before regenerating. Rose Tyler was too much the centre of his universe) with a burning passion. He was somewhat relieved to surface after regenerating and find that yes, he still loved her with everything in him. Last face that he saw before regenerating, he's managed to imprint Rose Tyler into his next incarnation (again).

But he's always wanted to take care of the people that he loves. If it weren't for the fact that he never really got physical with any of the (even Rose, regretfully) he'd say that he's kind of a sugar daddy with a group of collectibles (he blames Jack for the fact that he even knows what that particular twenty-first century idiom means)

He gave them what they needed, gave them what they wanted, and stepped back to watch them bloom in the world. He gave Donna a winning lottery ticket—because Donna Noble deserves nothing less than a fantastic life, even if she can't ever remember him in it.

Martha, who would never in a million years have accepted that sort of charity, well he got Martha a job. UNIT needed someone like Martha Jones, and he still thinks that.

He gave Amy and Rory a house—a house with a TARDIS-blue door. His own indulgence. So that the girl who waited for him can always see his bluest-blue, magnificently alive time ship, whenever she leaves the house. Amy and Rory needed someplace to grow old together, someplace to raise a family (which didn't end up happening, because he had failed them—twice, first in failing to find Melody so that they could hold their baby girl, and then by getting Amy sterilized, because he had failed to protect her from the consequences of being one of the people that he loved, failed to keep her safe from the target that his love tended to paint onto people's backs)

And now they'll never see that house, not ever again.

Rose, well, he gave Rose himself. Rose didn't need money, not like Donna did, and she already had a job, so she didn't need his help with that the way that he had put in a good word for Martha.

But the most special thing about Rose Tyler was that she had never needed the stars. Sure, they had had the best of times. His two years travelling with her are the years that he can remember being most happy since before the Time War. But she hadn't needed the stars, or the planets, or the adventure. It had made her happy, and she had been so much fun to explore the universe with. At the end of the day, all that she had really wanted was him. And that was what had helped his ninth incarnation heal from his smarting, incredibly destructive, wounding rage. He had offered her the entire universe, and she had said no. Then she stayed with him anyway, and he had been astounded by how quickly he grew to need her, the same way that he needed his two hearts and his TARDIS.

He had given them the TARDIS coral anyway, thrown River Song's stupid words back at his duplicate's head—The Doctor, and Rose Tyler, in the TARDIS, as it should be—and tossed in the name that would always belong entwined with his. His duplicate had nodded—message received. Take care of her, because I don't care what that little archaeologist said—I will never stop loving her.

And now, here they were, at the end of yet another story. Donna had lottery money, and Rose had him, Martha had a job that she was very good at, and his Ponds had a house that they would never see again, and they were beyond his reach now—even to take care of.

Even as he promises to show Clara the universe, some jaded part of him wonders how he will take care of her at the end of their story, what she will prove to need—and whether or not he'll be able to get close enough to her at the end to give it to her.