I don’t know why this occurred to me, but I just saw something about Omega and it reminded me of how the Doctor looked up to him as a kid. And here’s the thing.
The Doctor always knows his companions will go eventually. But he never seems to expect them to leave, or to find something important to do. He expects it to fall apart. He expects to lose them in some horrible, heart breaking, probably fatal way.
And why? Because all of his heroes let him down. Omega. Morbius. Rassilon. He looked up to them as most of his companions look up to him. He expects that he’ll let them down, because in his head, that’s what heroes do.