I love you...he knew it was true. He knew he wanted to say it, for Rose to know, and to love him back. But someday, she would leave. And the Doctor, even with all those years and memories that only showed in his eyes, didn't think he would be able to bear it. Rose was young, and the Doctor thought that she couldn't possibly love him the way he wanted her to.
Rose looked down. Maybe he was going to say it. Those three words, said too much but not enough for her. I love you. Maybe he knew. Maybe they could make this work...but she knew when she grew old, he would not grow with her. He would only regenerate, a new man every time. When she was gone, he would have to go on without her. Who would hold his hand, across the universe? Who would love him as much as she knew she did?