Entire galaxies before the Doctor, and he chose instead to study his companion's reaction.
The Doctor was old, and had seen many things - perhaps too many. But the one thing he never grew tired of watching was the completely awestruck expression of one seeing those things for the very first time. It was beautiful, and charming, and it renewed his passion for exploration.
There were still many things which the Doctor had yet to have the pleasure of witnessing. And while he may be able to visit each location at any time, he would never have the ability view each spectacular display through each individual being's perspective. He would never be allowed to see the joy lighting up every face that ever had existed, or would exist. The most spectacular sight: a living, sentient creature, relishing the experience of living, and all that the word entailed.
All of that beauty, evident in Rose Tyler's face as she stood, completely unmasked, looking out of the doors of the TARDIS. There was no pretense, no hiding the pure joy that radiated from within her.
And though the endless starscape before them was mesmerizing, there was nothing quite as fantastically stunning to the Doctor as Rose Tyler's face as she looked out into it.