The Doctor looked at Jack like he was both a blessing and a curse.
Jack couldn't help but fight his own emotional battles with the Doctor. Part of him was so angry at being left behind, and another part was so happy at having someone who knew what happened to him. But there was yet another part, a much larger and overwhelming part, which was grief-stricken for the Doctor.
His previous self was always so hard and serious. The only one who could break him out of it was Rose. This version was younger, much more pretty and cheeky (to Jack's delight), but there was a shadow that encompassed his eyes. A different one than the one that existed before. He knew its reason for being there.
Jack was so happy to hear that Rose was okay. Albeit, in another universe…a parallel one at that. From the look on Martha's face, however, Jack could see that the Doctor didn't allow himself to talk too detailed about Rose. He knew it hurt to even think her name, let alone say it.
Or maybe the Doctor wanted to talk about her with someone who knew her. Only then could anyone understand how much he lost when he lost her.
When Jack came back (heh, rhymes) at the end of the third series, he and the Doctor have that scene alone where they talk about Rose. The look on the Doctor's face and the sound of his voice made me think that if there was anyone that he could really talk to about Rose…it was Jack.
Cuz, you know, Jack loved her too.
But the Doctor needed her differently than anyone could need Rose.
*sigh* getting myself sad about the-Doctor-and-Rose lol hope you enjoyed.