“The Doctor’s death doesn’t frighten me. Nor does my own.”
Doctor Who never fails to remind us again and again the value and fragility of life. Perhaps what really hits home, is the idea that death is inevitable. We cannot escape death. You can run and run, but however hard you try, you can’t run forever. And River understands this: life is defined by death. For someone like the Doctor, who can turn around an army at the mention of his name but is powerless against the ultimate equalizer, perhaps there is an even greater significance.
Everybody knows that everybody dies. And nobody knows it like the Doctor.