You both are a couple who hide your identities as vampires to each other.

One night, you accidentally injured your neck, he sniffed your neck and his eyes turned bright red. You could tell that he's a vampire and seem so desperate to drink your bloods. You let him to drink on a condition that you will also drink his blood through his wrist, he was shocked but nods

You two drink each other's blood, he then licked your neck to heal the wounds and said

We're now linked honey, I love you