So I see a lot of people around going “Who was that dude the Doctor called in the final?”
“Oh, so he was an old friend of the Doctor’s. He went to his death after finding out that his old friend was dead.”
But it’s more than that.
The Doctor doesn’t have ties. He doesn’t go back and see people or keep contacts; he is completely and utterly disconnected from timelines and places and lifetimes. Except for the Brig. The Brig was the Doctor’s connection to Earth and normal linear-arlity in the same way that the NewWho companions stayed connected to their familes. He had all of time and space, and yet every now and then he would come and see the Brigadier - the heart of his UNIT family - always like how you go and see your old friends (only the Doctor would draw a load of trouble along behind him, naturally) and always (or pretty much, anyways) in the right order.
He’s got a time machine; they’ve got nothing on him. He can go gallivanting around with no limits. There is only one, tiny, cobweb strand that links him to a normal, grounded life where once a moment is gone it is gone and that is the Brigadier.