Yeah they never really explain WHY Death is bound to some magical macguffin like the crimson stone, but most other CV games seem to count Death as Dracula's confidante and friend. His ONLY friend.
If I recall, it's more based around the concept of Japanese death Gods that the idea arose that he isnt the grim reaper, but just "a" Death God. Or just some powerful necromancer much like Zobek.
of course I dont think the games themselves ever really went there.
I actually find it interesting that they can be friends, given how their connection works. Im guessing that simply after so long a time of being together they simply bonded. But now, if someone else theoretically acquired the stone, would that mean Death would betray and abandon Dracula?