There’s a whole vast universe to explore, not to mention all of time. The Doctor can literally go anywhere, and anywhen, he chooses. There are no limits. So it’s a mystery to him just why he keeps ending up on planet earth in the twentieth or twenty-first centuries.
“Castle? Why is there a big blue box with ‘Police’ written on it standing in your living room?”
“Search me. It wasn’t there this morning.”
The door opens and a head pops out. “Excuse me, could you tell me where I am? I seem to be lost.”
“Okay, I wasn’t expecting that!”