All vampires are lonely.
Legend has it that Cain, the first vampire in the world, fell in love with a human girl. Later, the girl encountered danger and her life was in danger. Cain made a blood pact with her and saved her. The girl fell in love with Cain because of this, and their lives were very sweet. But Cain did not tell the girl his true identity. Eventually the girl knew Cain's identity, and while Cain was hunting... The girl eventually left Cain and married someone else. But Cain slept in a foreign world and lived in loneliness all his life.
Since then, in addition to the six commandments of escaping from the world, dominion, descendants, responsibility, guest respect, and killing relatives, the vampires have also had a seventh commandment: companionship.
All vampires cannot take foreigners as partners, especially humans. Anyone who violates this will be severely punished.
Before Cain fell asleep, he placed a curse on all vampires: when you meet the one you love most, your heart will beat for him. If you can't keep him, you will sink in eternal loneliness...