The Things He Never Got To Say.
He'll never get to tell her that she saved him. After the time war, he felt empty and her smile brought him back to life. She is the only one that can make him giggle.
He would never get to tell her that giving her a hug after being reunited (or even for the hell of it) was the best moment of his day. That holding her tight and feeling her heartbeat against his made him weak at the knees.
He would never get to explain that he brought Mickey aboard the Tardis to help him keep his distance from her. But with him being there, it turned him into a green-eyed monster. Time Lords are not supposed to have such a "human" emotion but she brought it out in him anyway.
He would never get to confess that he made a mistake with Reinette and that he was sorry. He didn't just cross the line; he took a running jump over it.
He wished that it was her that kissed him, not Cassandra. And the kiss on the Game Station and Rome was not enough.
He should have told every day what she meant to him and have never wasted a second holding back. All the times he could have just said it. Just to let her know that she wasn't just his best friend.
He never got to say "Rose Tyler… I love you".