"I love you."
It is one possibility out of a trillion possibilities that Jack would end up as companion to the Doctor in his tenth regeneration. It's a possibility that is healthy for neither of them, but they both have a history of choosing many things before health.
In this universe, after Rose and before Martha, the Doctor finds Jack again. Maybe it is guilt that drives him, or simply forgetfulness about the things that are bad for him. He has lost so many things that he often lets safeguards and reminders slip through the cracks when he ought not do so, for his own good and that of his companions.
But Jack has a nice coat and strong hands and the Doctor needs him, although he doesn't trust him. The Doctor is good at putting his own needs before other people's, but he's also good at the opposite. He's not as complicated as he supposes, however, and certainly not as complicated as Jack thinks he is.
Now, Jack is standing beside the control panel of the TARDIS and mouthing the words silently to the Doctor's back, a faint smile of his lips. This is safe. This is all right.
"I love you."
When Jack says it, the Doctor isn't surprised, but he is worried. When he sees that Jack won't ever take it any further than a brief remark, though, he relaxes and can shrug if off easily.
He tells himself he can't say it back because there's only room for so much love inside him, but that's not true. If there's a limit, he hasn't found it yet. However, he does find his capacity to care limited in other strange ways, and Jack is one of those sticking-points.
And it's not as though the Doctor is new at this whole love thing. He's done it before, oh yes. Hundreds of times, probably. He just can't do it for Jack, not yet, maybe not ever. Jack is still broken and wrong, wrong in ways even the Master was not, and The Doctor cannot make everything better.
Besides, Rose is still there like a shadow on his mind, yet another wound that won't fully heal, no matter how many others he suffers. And there will be others.
The Doctor has never burned up so much as a cigarette for Jack and they both know there are reasons for that.
"I love you."
Jack says it offhand once, just for the hell of it. Not, of course that saying it like he doesn't mean it makes it any less true. It's just easier to deal with his emotions if he pretends they're someone else's. For a long time, love was something that happened to other people.
The Doctor turns to him with a slightly panicked expression, but then he catches a look at Jack's face and beams again. "Nah," he says, waving a hand.
And that's the end of it, for Jack. He's not going to push something he knows will fail, at least not openly. He vows that he'll never say it again. Of course, that only goes so far, and vocalizing his feelings isn't his main avenue of attack. He never promised himself he'd get over the Doctor.
There are stars out there, spinning past the sides of the TARDIS like so much dust, but Jack tries not the dwell on that. It just makes him homesick for something that was probably never his in the first place. Instead, he focuses on the Doctor and the look in his eyes when he sees fire.
It feels like home.
I love you.
The Doctor has heard the words a hundred times, more. Some of them meant it and some of them only thought they did, but all of them were completely terrifying when they said it. Their eyes always burned with such intensity, and the Doctor has learned since to fear fire.
Also terrifying is the extent to which they'll maintain their love. It endures, human love, no matter how many things the Doctor does that should dissuade them from feeling it. Even Romana and the Master took a surprisingly long time to give up on him. He hopes they did, though, in the end. The hope that always burned alongside the fire was worse.
Jack is different, because he's missing the hope entirely. He laughs because he has to, just like the Doctor does at times, but he does it without the sense of delight the Doctor still manages to drum up.
It scares the Doctor, honestly. He's not sure he can love someone so grim and hopeless. It might begin to rub off on him as they always do, and he couldn't afford that at this point. He's already too close to the edge. Jack must understand.
I love you.
Jack has said it to the Doctor only once, but he's also said it a million times, in other ways. Most importantly, he says it when the Doctor offers to let him leave, just drop him off here or here, and Jack says no. No, not this time.
Every time he says it, he can see hope and fear bloom on the Doctor's too-expressive face. Hope because Jack has been a constant. Fear because every time he says he'll stay, he's one step closer to leaving. He will prove the Doctor wrong about that if it takes a thousand years. They've both got the time, after all.
Yes, they've got time, and nothing but. Every time the dirt of a new world crunches under Jack's feet, he smiles because he knows they'll leave soon enough. There's nothing in all these brilliant worlds that can have a permanent effect on him. Nothing, of course, but the Doctor. Jack always remembers this and stops smiling immediately.
There's nothing to do but go on, though. They keep traveling through the vortex, burning time until Jack can find a place where the Doctor can say it back and mean it.