I found this article fascinating. Dr. Bruce Goldberg explains very well how energy vampires draw their victims in and drain their energy. Keep in mind that there are other views about what energy vampires are, and I don't agree with all of his conclusions. Some of his insights are very interesting and seem quite likely, especially in respect to these particular kinds of people. Rather than state here which parts I agree with, I'll simply share the article and let you decide for yourself what has merit within it.
Energy Vampires
Dr. Bruce Goldberg writes:
Some people, by their very presence, seem to drain the energy of those unprotected people around them. They will often associate with a victim who exhibits the classic signs of this drain.
General debility, lack of motivation and energy, an emaciated physique, a pallid complexion, and an overall sense of weakness are typically noted. These victims also tended to be highly suggestible people. The dominant partner (energy vampire) always resists a successful treatment, or protection, applied to their victim.
Most of these energy vampires are well-meaning, normal people. They are an unhappy lot, and do their damage by a telepathic draining of their victim's energy resources. Mere separation of two people results in immediate positive changes in the victim. In my Los Angeles office I train all such victims in the art of spiritual protection techniques to eliminate any recurrences of this syndrome. Try the psychic protection negativity experience on the hypnotic exercise link for your own protection.
I prefer to use the term "psychic parasitism" for cases where this energy drain is involuntary and subconscious. If this effect is the result of a premeditated psychic attack, I refer to the perpetrator as an energy vampire.