Disclaimer: I don't own Doctor Who. That pleasure is all the BBC's. I do own Darren, although I'm not sure that's something I'm proud of...
Every time he closed his eyes the Doctor was haunted by the same vision. He was taken back to the day he left Sarah behind.
"Doctor, please," she begged him. "You can't just leave me."
"I'm sorry, Sarah," he replied. "I have no choice. I can't take you with me."
"Alright," she said. "If you can tell me that you don't want me with you anymore, then I'll go."
He met her gaze and uttered the words that broke his hearts. "I don't want you with me."
He had abandoned her because he was a coward. He was afraid of his feelings for her. Afraid of love.