Doctor Who’s Quinquagenary → Companions
“You will have realized, of course, that you’re not the only human who has traveled with me in the TARDIS. The Time Lords have often wondered why I bothered. After all, we are capable of living for thousands of years; you can barely reach a hundred. And they came up with a theory. They thought you were all memento mori. Reminders of death. Quite common things really. On Medieval Earth, courtiers would often keep skulls on their mantelpiece. No matter how powerful you were, death was inevitable. You still had to remember your mortality. Time Lords need to remember all the more. I denied that was the reason of course, and as you said, friendship, companionship. But over the years, over my many lifetimes, as my friends all left me one by one, I began to wonder that they really might have had a point after all.” –Eighth Doctor, Scherzo