Jackson Lake: Tell me one thing. All those facts and figures I saw of the Doctor’s life, you were never alone. All those bright and shining companions. But not anymore?
The Doctor: No.
Jackson Lake: Might I ask why not?
The Doctor: They leave. Because they should. Or they find someone else. And some of them— some of them forget me. I suppose in the end… they break my heart.
— Tenth Doctor, The Next Doctor
This is such a sad quote (especially if you know the story behind “the one who forgot”), but I love what it explains about the Doctor’s life and the problems that come with living a long life. It also explains why he sometimes decides that he should travel alone, which is never actually a good decision. When you lead the type of life the Doctor lives, and you live it for as long as he does, you inevitably lose people. And this Doctor, who is funny and clever, and has more adventures than you can count, also gets his heart broken time and time again. But ultimately he always finds someone new to travel with. Because he has to. Because it’s not good, especially for him, to travel alone for any great length of time. He loses perspective, and sometimes he gets carried away. Sometimes The Doctor needs friends to remind him of what kind of man he is.