What is so fascinating about vampires?

What is is that's so fascinating about vampires? They're monsters, but they're almost human, and they used to be human. They're powerful. They are all but immortal. They are creatures of darkness, confined by a cage of daylight. They take someone else's source of life as their own. They are ultimately selfish and soulless, and they do everything for their own pleasure. If someone else benefits from their actions, it is because it benefits them also... Maybe it's because they're so close to being human, but they're not quite. They're a twisted version. What humans would be were they completely evil. They have human appetites outside of the bounds of morality.

Or maybe they're simply scary monsters. I would just like to see a more complex reason for why I love shows like Buffy the Vampire Slayer and Angel and the notoriously awful movie Van Helsing.