It's not a habit of the Doctor to visit old companions, except when they call him. Maybe it's a lingering anxiety. If, in their future, he was to learn something happened to them, he could no longer do anything to stop it without breaking the laws of time.
Maybe he likes being brought back only by fortune, on the wings of a tempest, rather than risking turning up after it.
But this time, Astrid's death reminds him of everyone and everything he has lost: the Master, Rose, his whole planet. He finds himself needing to talk to someone. Seeing his former incarnation awoke in him a wave of nostalgia, so he sets the coordinates of the TARDIS to Terminus, about five years after his last visit.
The corridors are so clean and well-equipped that he almost doesn't recognize the station. He meets a few people; they look healthy and don't seem shocked by his presence. Feeling optimistic, he proclaims in front of the first surveillance device he sees: "I'm the Doctor, and I'm here to see Nyssa of Traken!"
(And he gets ready to run, just in case.)
It takes only a few minutes for her to appear in the hallway, one of the Vanir at her side.
"Nyssa!" She smiles, almost exactly the same. He runs towards her and takes her hands. "You made it quick!"
"Our sensors spotted a TARDIS, and I came as soon as possible. Oh, Doctor, is it really you? I thought I'd never see you again. I heard that..."
He interrupts her. "So all of this is your work? Isn't it magnificent? Oooh, it's a real hospital now! You have the equipment to cure a whole bunch of other diseases, I see!"
"Since the cure works, the Lazar disease became less and less frequent," Nyssa explains calmly. "Moreover, as the patients can go home now, we have all the room we need."
"Impressive! It's all set, next time I'll catch the flu, I'll come and get healed here! Do you have a little shop? I'm sure you have! Is the Garm still here? You absolutely need selling Garm plushies!"
Nyssa smiles when she looks at him. She answers his questions with a quietness that's in contrast with his restlessness. She listens with interest to his suggestions - at least, the ones that don't involve plushies.
Only when they're alone does he drop his smile.
"You regenerated," Nyssa murmurs.
He interrupts her again. "You heard about the Time War, didn't you?"
She nods. "People come here from all around the universe. I heard... about Gallifrey. But you survived."
"Yes," he replies. "I'm the only one."
There's a long silence. Nyssa puts her hand in the Doctor's. He holds it tightly and remembers why he came to see her.
"How could you bear it, Nyssa?" he asks with despair in his voice. "You lost everything. You were the last of your family, of your planet, of your civilization, and you never complained, not even once. Of course, I knew it was hard for you... but I couldn't understand how much. How did you do it?"
She seems taken by surprise. "I helped people... like you do." She lowers her voice, like it is a secret. "And I had you." Then, hesitant. "And, if it can matter, you have me."
Maybe she just intended to kiss her cheek, like when they said goodbye the last time. She doesn't seem startled at all when their lips meet.
She blushes afterwards, though, looking at her feet. "Not only me. All of us. I always think of you, and I'm not the only one."
He grins. "I'm getting old, Nyssa. Sometimes I need to be told the same good things over and over again."
Nyssa smiles too. She is still infinitely stronger than he is, and he knows it won't always be enough.
But, just for today, it is.