Good things never last and no one knows this better than the Doctor.

He's nine hundred years old and he's watched cities fall to ruin, companions wither and leave, planets collapse—

Gallifrey.

—and always, always, he's the catalyst.

Gallifrey and the Time Lords and the Daleks but in the end it all boils down to the Doctor. The Doctor did it.

He is fire and ice. He is the Sun, bathing everything in its light, but be careful not to get too close, because he also brings death and destruction.

Murderer.

The Doctor knows this. Whatever he touches, whoever he befriends—they all fade, in the end. Oh, sure, he shows them the universe and beyond, he lets them touch the streams of glittering stardust—the stuff dreams are made up of, he blurs the line between fantasy and reality, all while dancing around the TARDIS' console in an explosion of brilliance.

But it doesn't matter, because no matter what he shows them, they always leave. They perish. They die. And when they die for him, that's when the Doctor tells himself this is all he's capable of bringing.

Death.

Ruin.

Emptiness.

Annihilation.

Extermination.

Sometimes, he thinks, he's no better than a Dalek.

So he tells himself no more companions, because it'll be better for them both. He tells himself he's got the TARDIS, and the old girl hums in agreement because she'll always be the one permanent fixture in his life. He tells himself he can travel on his own and he doesn't need to wreck any more lives by bringing them into his maelstrom of destruction.

And for a few days (or trips, since it's hard keeping track of time when one is traveling in the TARDIS, even when one is a Time Lord), he pulls the I-travel-solo card off.

But humans are never meant to be alone forever, and in that regard the Doctor is more human than alien.

He gets lonely.

So he strikes up a conversation with this waitress or that scientist or that journalist, and all too soon he has deluded himself into thinking that the last one, this'll be the last one I'm taking with me.

That's a lie and he knows it, but when the TARDIS is filled with laughter and another voice besides his own, he can't bring himself to care. The time he spends with his companions is painfully short—a mere eyeblink in the life of a Time Lord, so he treasures each one and he's so very careful not to blink.

When they go—as he knows they'll always go, because he's the Doctor and he always makes good things go away—the absence chips away a little piece off both his hearts. He flies on his own for some time, and the TARDIS feels exceptionally quiet when a companion has left, and he gets lonely again.

So he takes someone else with him, has his eyeblink of company, and watches them leave. Each time he tells himself this is temporary, temporary, because when you know what will happen, it prepares you for the hurt. It numbs the pain.

Well.

Theoretically.

It's not true, though, and the Doctor knows it. Each time they leave, the void gets bigger, and all he can do is find a new companion to take his mind off the silence, but it's temporary, temporary. It's a vicious cycle, one the Doctor can't break.

He knows they'll have to leave, because they are good things and good things never last.

Now he's standing next to Rose Tyler and they're watching the Pterodactyls soar across the bleeding sky. He's holding her hand, and he might just be exaggerating but her hand makes for a very good anchor that keeps him grounded. Just her presence, her thereness, is comforting. Right here, right now, they're just two beings, two minds, two hearts—well, three, actually... but he's so content the words slip out before he can shut his mouth.

"How long are you gonna stay with me?"

She looks at him, smiles, replies with certainty. "Forever."

He's too giddy to realize what that one word implies. Foreshadowing of the cruelest irony.

Rose Tyler is a good thing.

Good things never last and no one knows this better than the Doctor.

But for that wonderful, fantastic moment, he lets himself believe they do.