Emotional vampires exist out here. They have no independent sense of absolute happiness, rather, they are only happy when others are not as happy as they are. This can be a problem when they are miserable people to start with, as is often the case. As soon as an intrinsically happy person enters their orbit, they latch onto them and suck out their joy, methodically, parasitically, ruthlessly.
On the other hand are people with savior complexes. These simple idealists seek validation in praise for their good deeds and place great faith in the strength of their virtue and the purity of their intentions. They think they add value to others' lives by simply being there. They know they are walking good luck charms. They deserve the best, and if they settle for less, they are doing someone a favor.
These two, the vampire and the saint, gravitate towards one another to their mutual misery. One gives and gives and is never satisfied with returns on investment, and the other takes and takes and is never satisfied. Their only way out of the pit is more digging. A whole pile of dysfunctional psychological schema accumulates around this basic pathology of misery, a toxic environmental eyesore, but the fear of separation dissuades both from ending the relationship, which often gets stuck in a vicious cycle of revenge, comeuppance, power play, neglect, resentment, frustration, infidelity.
And for what?