Jack had hoped when he eventually found his Doctor again the Time Lord would be able to fix him, make him a normal person again, with only one life to live just like everyone else.
His hopes were soon dashed.
The Doctor said he was wrong, an impossible thing, but for all his disgust over what Jack had become, there was nothing he could do. Jack was a fixed point now, doomed to live forever; he’d never get to spend his life with someone, because no one could match his longevity.
The universe would grow cold and he’d live on.