Replacement
It's been three months to the day since she died. Three months, and the Doctor has been thinking about her every single minute. Jack knows. He's been watching.
He knows the distant look the Doctor gets sometimes, when he'll stop suddenly and stare into space – sometimes only for a few seconds, sometimes for hours on end – forgetting everything else around him. He knows the faint half-smile of a fondly remembered memory, quickly fading into silent grieving. He knows exactly how difficult it must be for the Doctor, because he feels the loss every bit as strongly.
...Perhaps not as strongly. He's not sure that anyone could miss her as much as the Doctor does. But still he sees the pain that the Doctor is going through, and he wants to ease it.
Sometimes he wants to take the Doctor by the shoulders, wants to look into his eyes and kiss him and say look, you haven't lost everything, I'm still here, surely I must mean something. He can never be what Rose was, but he loves the Doctor and the Doctor cares about him, and that should be enough. I want to help you. Let me in.
But he never does.
He's not used to these emotions. He can deal with danger and sex and explosions, but this is something different. He doesn't understand what to do now.
He's afraid that he won't be what the Doctor needs. He's afraid that his well-intentioned advances will only serve to distance him even further.
Eventually, though, the Doctor breaks down. Whispers hoarsely that he could have saved her. Should have been able to. Should never have been so stupid.
Faced with him like this, so quiet and broken and somehow helpless, Jack has never wanted more to kiss him. To try to make things better.
So he does, and he is almost frightened by how desperately he responds.
That night he kisses him and trails fingertips over his bare skin and calls him Rose, and even though he knows that the Doctor needs this outlet, Jack has never felt so unwanted.