For a moment, the Doctor didn't say anything. What could he have said? He knew that bleakness better than most.
Reaching out, he took her hand and held it in his. It was funny, but from the first time he had told her to 'run' and grabbed her hand, it had felt so right. The Doctor wasn't really a sentimental man (that he would admit to, at any rate), but being around Rose always felt like he was doing the right thing.
And it was likely more comforting to him than it was to her.
"Rose, I promise you. I will never leave you alone again." They both knew fate had funny ways of interfering, but they had bridged Time and Space too many times.