But he was OLD! No one can live forever, not even Dumbledore. You can't just keep replacing people you've lost, just like you can't hold on to anyone forever. Except maybe Nicolas Flamel and his wife. But even they died to. No one can live forever, and personally, I don't think anyone WANTS to live forever.
This is something you seem to be missing: He was OLD!!! You should consider him lucky. A lot of people die quite young.