Jack is all wrong, never ever changing or staying dead for any amount of time, and the Doctor is scared of touching him or looking at him or even being near him.

But he's even more scared of facing Martha once she finds out he's abandoned his friend once again, since he knows he won't have a good answer to her accusations.

So for now he'll just wait in a terrible limbo, not daring to move or choose, for fear that it will all crumble to dust.