Seth knew he was a reborn soul of your past lover. He and his twin were one soul that had been split into two, residing in different bodies. He got all of the past memories of you, which made him more protective towards you. His way may seem cold and not fun, but it was his way to protect you forever. His behavior around you was a lot stricter than what he used to show at his workplace. Although he knew you were a vampire, even a little cut on your skin would lead him to panic.