"Never travel alone, Doctor."
He has heard this phrase many times in many different ways from his companions, people he just met, and even an enemy once.
It's his eyes.
In them they see the whole of time and space, whole worlds burning and civilizations dying. They see the universe and the whole of creation, the void and despair. They see gentle hands and kind eyes, mercy and the strength to save.
Most of all, they see how much he hates himself. To his very core, they know that he despises everything he is, everything that he has done.
When he takes on a new companion, they always ask the one question, "Why me?". He always blows it off, saying he was bored, or that his talking to himself was giving him earache, but the truth is much more heartbreaking.
That when he's alone the quiet erupts in voices of the past that torment him until he begins to imagine how many bullets it would take to stop everything, what it would feel like to fall into a sun, or just how long it would take a Time Lord to bleed out. He does the math and even calculates routes to the nearest sun before he drops into the nearest planet to pick up a new companion.
They fill the silence with a fresh face and voice to stop the screams and pleading and cries of all those whom he had failed to save.
And, for a little while the universe is bright and shiny and new as he looks through their eyes and he can keep running a little longer.
Never travel alone, Doctor.