The TARDIS needed a window. Why was it that he had this magnificent ship with innumerable rooms that could hold up under atmosphere re-entry, withstand hellish temperatures, fall from cliffs and survive attack without a scratch, yet there wasn't a window to be found? He could do anything he could possibly imagine in the TARDIS – except watch the stars and the moons twirl around his ship. The only time he really got a chance to see the universe lit up it all its grandeur was when he was trying to save it. And to take the time to stop to ponder or appreciate his surroundings in those moments could mean the end of them all.
So, why were there no windows? The Doctor had decided it was to remind him of his solitude as the last Time Lord. Regardless of the number of traveling companions he may collect throughout the galaxies, his world was just him and the TARDIS. Perhaps, if there was a window, his memories and experiences could escape into time and space and settle in the stars. There would be a little piece of him left behind somewhere that didn't actually involve saving the behind. He could rescue the universe time and time again as the Doctor, but it shouldn't be remembered that he was a being with a name and a story all his own. He was destined to travel time and space locked - no, sealed - in the soul of a spinning box. A soul with no windows.
"Hey, Alien Boy!"
He saw a flash of red hair outside the library door. His companion walked by without noticing the extra door, and he didn't call out to her. The Doctor sat behind a large dark wooden desk covered in books and random papers. He had his chin propped on folded hands as he rested his head on its surface. The organized mess appeared to be rather serious in nature. However, were one to take a closer inspection, one would see that the books were how-to-manuals. How-to-impress-your-friends-with-your-elementary-art-skills. How-to-cook-a-meal-that-won't-make-your-companion-choke-on-her-own-vomit. How-not-to-bore-everyone-around-you-with-usless-historical-drabble-when-the-drabble-is-chasing-you-with-a-flaming-tentacle-head.
However, one little book stood out among all the others. It had been read by only one and would be read by only one other, but he couldn't imagine that a more important collection of thoughts existed in the world. The Doctor had been staring at it for weeks. He'd carried on life as usual, fighting this and saving that. But as soon as the TARDIS got quiet, he could hear the little blue book vibrate with all the secrets of his universe. He knew that to open the book would be a violation of his own rules, and how could he impose rules on others if he was not willing to follow his own?
So, he periodically visited with the book. It had become a companion of sorts, in its own right. Except the book reminded him how truly alone in this world he was. He didn't know how, when, where or why his future was filled with the contents of the book. But he knew who. And he knew that the discovery of the how, when, where and why would be the greatest of all his adventures.