The Doctor was dying, slowly but surely. It wouldn’t be a permanent death, as a Time Lord he still had a few regenerations left, but it would be the death of this particular body, which was a shame. He’d kept it longer than most, and experienced a lot of adventures; it would be hard to let go, not knowing what kind of person he’d be next.
It was comforting to know he wouldn’t be forgotten though; he was leaving behind a legacy in the form of the people who’d travelled with him. Maybe he could visit them one last time.