Parenting
He was too old for her. She did say he wasn't her boyfriend, but it was clear that there was somethingthere. They might not know yet, but if there was one thing that Pete Tyler knew about was not wanting to admit one was in love (after all, it hadn't been easy, saying he had fallen head over heels for Jackie).
It was in their eyes, in the way he complained to her.
In the way he sacrificed himself so they could live.
In the way he wanted to avoid his death, so Rose wouldn't have to go through it again, being an orphan.
The Doctor had wanted to protect her and even when he was mad, couldn't punish her. He would made a lousy father for Rose, but she was too grown up for that - it wasn't that kind of love.
The Doctor had given him more than he could have dreamed, a chance to make it right, to be a good father when he would never have been one otherwise.
He would be good to Rose, even with the age gap, he didn't need to worry.
He didn't need to ask The Doctor to look after his daughter, he didn't have to wonder if she would be happy. He knew that as long as they were together, they would be alright.
He could just walk and meet his fate - save the world, save the Doctor, save Rose.
And die.