The vampire is the most obvious metaphor for the emotional parasite. From Buffy the Vampire Slayer to Bram Stoker's Dracula- we have been telling stories about those beings that live off the energy of others.
We are both attracted and repulsed by the image of the vampire. There are the sexual undertones, the crossing of boundaries and the appeal of an eternity lived half dead.
We identify with this being from our own lives. We all know people who prey on others. We may have at some point been victim to such people. Whether that be bullying in school or the workplace, we can identify the vampires that live off the energy of others.
We are implicitly aware that vampires are only half human and have lost their souls. In concrete terms, this means that the vampires are not living their own fulfilled lives.
There is also the issue of deception. We are not always aware of who the vampires are until they strike. They may seduce their victims through promises or praise until it is too late. In a close relationship, flattery can be used like a drug. When we come to depend on the praise of others, this can be withdrawn at precise times so that the vampire can influence and control the victim.
As always- the lesson is to remain one's own most trusted friend. To be adult means making your own decisions, trusting your own thought processes and not relying too much on the friendship of others.