Four times he came so close to saying the unthinkable.
"I don't age. I regenerate. But humans decay. You wither and you die. Imagine watching that happen to someone you..."
The First Time: He never meant to hint that he cared more deeply for her than he should. He couldn't let himself fall, because everyone he ever traveled with had to leave at sometime. As much as he wished, there was no way she was an exception to that rule.
"If we get back in touch—if you talk to Rose, tell her… tell her… oh she knows…"
The Second Time: He was vulnerable. Death and Satan and discovery were waiting for him, and he thought he would never see her again. His beautiful, strong, pink-and-yellow Rose would be stuck on that planet alone. In that split second, he decided he wasn't ready to die. He wouldn't leave her up there with no escape route and no way home and he couldn't possibly leave someone else to deliver his message. He would fight his way back to her so that he could tell her himself. Then, he let go and plunged into the unknown with new-found strength.
"And I suppose, if it's my last chance to say it: Rose Tyler…"
The Third Time: He was saving her. Repeating that in his head was the only thing stopping him from bursting two universes open at the seams just to be able to touch her one last time. He was letting her go, because a Rose Tyler that he could never see again was better than a dead Rose Tyler. He knew that this was the end of their adventures. This was the end of them.
Their limited time was ticking as they stood so close, but so far apart, on Bad Wolf Bay. She was sobbing, and he was fighting to keep control. Then, she said they one thing that he thought no one would ever be capable of… she said she loved him. After all the devastation that he caused— the genocide that he felt even now weighing on his shoulders— he wondered, "How?" He couldn't possibly let her see his inner turmoil, so he hid it under clever remarks.
"Quite right, too."
He pauses before murmuring her name, and the connection breaks before he can finish his sentence. He's back in his time machine, alone… again.
"Does it need saying?"
The Fourth Time: She was back, breathing, and in a universe where she no longer belonged, but she was there with him. He was beyond ecstatic and only slightly worried about the impending collapse of the cosmos.
Then, out of nowhere, a duplicate of himself is running around with Donna's loud attitude and a timelord's knowledge of time and space— his knowledge of time and space to be exact. A deadly mix to begin with all crammed into a human body.
Flash-forward, and he found himself standing on Bad Wolf Bay— that god-awful, ironic place— again, with memories flowing back into both his head and his double's. Instinctively, they both inched closer to the woman they thought they lost. He knew he couldn't stay and the duplicate must have been able to see the surrender written across his face. Rose was aggravated and seething, looking for answers to questions that he couldn't bring himself to acknowledge.
He could tell that she needed the reassurance of his response to that sentence long left unfinished, but she didn't understand how hard this was for him. If he had said the words that still haunted him, he never would have been able to walk away from than beach willingly. He would have ripped the universe apart looking for a way to stay, but his rational side knew it was impossible.
So, he left the task to the meta-crisis. He watched as the words taunting him were whispered into her ear. He stood by silently as acceptance and joy flittered across her face.
He left the beach that day not because he was forced to, but because he knew that it was right. He let her go… left her with a part of him that could love her until it died a normal human death. He left with his words still unsaid.
He was dying, plain and simple, but there she was… backing two-thousand five, before all their adventures began— before he even had this face. There she was all pink-and-yellow and barely nineteen, an entire lifetime ago, when she was just a shop girl. He knew her inside and out. He knew everything that she would be, but this version of her didn't know him yet. He settled for a light conversation, it was New Years after all. And then after promising her a spectacular year, he let her go one last time, because she was the girl who slipped so easily in and out of his life. He loved her anyway.