The doctor had known loss, had known pain, had known loneliness and anguish, after all he had lived 900 years.
Living that long, he's lost more than his fair share of companions.
But no loss, no pain, had ever cut him as deep as the loss of the Ponds. Amy and Rory were gone from him, disappeared as fast as a breath on a mirror.
He was absolutely gutted, sitting on the steps in his Tardis, staring straight ahead, not knowing how to live with the rest of his days.
His eyes rested on the curly-haired girl pulling levers and pushing buttons on the control mainframe of his spaceship.
River Song, a dear friend, and half time-lord, was flying the Tardis away from the weeping angels.
Eyes crystal blue, feeling his gaze, fixed back on him, she then told him in no uncertain terms should he ever be alone. He needs to find someone new, someone to travel with. A man like the Doctor should never be alone. Never.
He offered her to travel with him, to live adventures anew, across the cosmos, yet in a lackluster way. No one could ever replace the Ponds.
She smiled at him in the characteristic River Song way; endearing with a touch of condescending.
"Anytime, anywhere." But she added "Not all the time though, only one psychopath per spaceship don't you think."
His heart sunk a little further, not surprised at her answer.
She told him, that she should write the book and send it to Amy. The book that started the whole adventure, the mystery that took away the Ponds.
Yet she had to do it, because it was a fixed point in time, and she couldn't screw around with that.
He agreed quickly, not wanting to talk about it anymore. He tried to wave her off.
She told him then, as she was leaving, that she would ask Amy to write an afterward, for him. Without looking back, she walked out of the Tardis, and back into the past, where Amy and Rory lived.
He pondered her parting words, and realized, he had ripped the last page out of the book, claiming he doesn't like endings.
He never read that afterward.
This understanding dawning on him, propelled him to fly the Tardis back to New York, back to that boulder they had once lunched on, in Central Park, back where he left that paper sitting atop the basket, upon that boulder.
He found the yellowed page, and promptly put his glasses on and read.