I’ve always thought that there is a calm place for the Doctor in the TARDIS. He probably couldn’t find it if he tried, but whenever he’s aimlessly wandering the halls fatigued and melancholy, he tends to stumble into it. It is her way of helping, in between companions.
The room appears to be a garden with a personal universe. The planets hang in the air, and if you watch closely, you can see them orbiting. It’s quiet, and lit only by the local sun and the glow of some of the flora.
Though he tried to hide away after the loss of the Ponds, he always seemed to find himself in this room again. It’s never quite the same, but he wouldn’t have it any other way.