As a vampire, you must accept that every person you ever know will die, and you are the only constant in your life, the only person you can-and should-rely on.
The quote emphasizes the profound isolation that comes with being a vampire. It suggests that a vampire must come to terms with the transience of human life, as everyone they form connections with will ultimately pass away. This reality forces them to confront the nature of their existence and the inevitable loneliness that follows.
Moreover, the quote highlights the idea that in the face of perpetual loss, the vampire must learn to depend solely on themselves. It suggests that self-reliance becomes not just a necessity, but the most trustworthy aspect of their eternal life, underlining the emotional toll of eternal life and the solitude that accompanies it.