He was the owner of time. He controlled time. But his time with Rose Tyler had run out. Every time he closes his eyes he sees her out stretched arms falling into the void her mouth open in a scream. She's reaching for him and he's reaching for her the only thought in his head is, "please don't let her fall into the void."
Pete pops up and grabs Rose before she can fall into the void and then they're both gone in a flash. He's standing outside her room now. He has no idea how he got here or how long it's been since she was locked into that parallel universe. He sits on her bed. It still smells like her. Her clothes strewn haphazardly throughout the room.
Before they had left to her mom's flat he had told her to tidy up a bit, "Really Rose what would your mum say if she saw the state of your room?"
"I'll clean when we get back from my moms, I promise." Now that promise would never be fulfilled.
He's walking around her room running his hand over her things. The TARDIS seems to big to quiet without her. He stops in front of a framed photo of him and Rose, he has his arm around her shoulders and he's looking down at her. She's staring at the camara her arm around his waist. The picture had been taken no more than a month ago on some alien planet.
The Doctor studies his expression in the photograph. It's the same expression he had when he looked at Rose, but only now does he realize why he only looked at Rose that way, why she warmed his hearts like no one else, why everyone always told them that they were an adorable couple.
He loved her.
He's running to the control room then, pushing buttons and pulling switches he doesn't care if he has to burn up a sun he just has to let her know that he loves her.