They always leave.
The Doctor knows this. He's been through it too many times not to know this. Throughout all his regenerations, all his companions have been wonderful people. Magical, wonderful, fantastic, brave people. They know they have to leave too. Sometimes, he wonders if that's what makes them so brave.
But this time they hadn't left. He had.
Did that make him, the old cowardly raggedy Doctor, brave? If it did, he decided, he would rather not be brave at all, and have them back. But he knows he can't.
It hurts, to be sure. Maybe not as much as when (Rose) she had been lost, because at least the Doctor knows Amy has Rory, and Rory has Amy, and they have each other. Maybe not in the same way as Martha, because she had left him. And not at all like Donna, who has no idea any of it ever happened. At least they have their memories. But it hurts, to be sure.
He isn't brave. He's an old man, a very old man who wants to do some good in the universe to try to make up for the very very bad things he's done. Sometimes he wonders if perhaps he should just go it alone. Not have to deal with the heartbreak and the sadness and the hurt-
But he's too weak for that. The loneliness might break his two hearts, and then what would the universe do, without a Time Lord to protect it?
Not for the first time, he thinks of the Master. And promptly shuts that thought out of his head. If he wanted to break down crying, he would not do it over him.
But his eyes betray him, and judging by the way his eyes are stinging and his throat is dry and his face is wet he has been crying for quite a bit. A sob escapes him, and he bites his lip and shakes his head.
He'll just have to go it alone. He spares a thought for (Rose) her and wonders what she'd think of him, her Doctor, this new regeneration of him. She's the only one who knows what it's like to have two different Doctors. Would she have accepted a third?
But he can't think of this, think of things that can never be, will never be, and he's crying again, silent tears slipping down his cheeks. This is the first time in a long time he's allowed himself to think about his feelings, and his old companions.
Allowed may not be the right word though, as it seems more like the steel door he's put in his head in front of his memories has burst open, a flood of memories coming through. He lets himself cry, quietly, the TARDIS humming around him, and then wipes his eyes. (Rose) She wouldn't approve of this. Neither would Amy, for that matter. And so quickly shutting off thoughts of his Ponds, his companions, and his (the only person he's ever truly loved, his other half, the one whose name he never thinks of, can't allow himself to think of-)…well. He shuts off thoughts of them.
He goes to the TARDIS' console. He leans over it. Allows himself a glance at the sweater Amy's left behind. And he pulls a lever.
The Doctor sends himself anywhere into time and space, letting the TARDIS do what she will. And he floats, the poor, lonely, sad, old man, looking for a new companion, and looking to forget, even if it's just for a little while.