A monster is what people fear most. And what do modern people fear most? Loss. People are greedy, and cannot forgive losing things. But the problem is, material goods can be replaced. What cannot be replaced is the people you know, your friends and family, even simple co-workers and acquaintances.
Imagine, if you will, that one day, you go to hang out with your best friend. He seems confused when you greet him, seeming to not recognize you immediately. He asks you, confusedly, why you are greeting him, as you were just together, having a good time. When you get home later, there's a message on your voice-mail from your girlfriend, commenting on the great day you two had, reminding you that you are meeting for dinner later. You haven't seen her in a couple days, and you call her, finding out what's going on. She doesn't recognize your voice, and hangs up when you claim to be her boyfriend. Concerned, you go to her house, and knock on the door. You hear her speaking inside, and then hear her walk up to the door. The light in the peep-hole is blacked out as she looks through it, and then she leaves the door unanswered. You knock again, frantically, and she finally opens the door. Worried, you ask why she ignores you, and she angrily claims to not know you, and slams the door.
Miserable, you go back home. You can't find your key, but the back door is unlocked, so you enter through there. Your roommate is in the room you enter, and doesn't recognize you. He shouts at you to leave, that he'll call the cops. Suddenly, someone else enters the room. This person finally does recognize you, but all he does is give a sly wink, and then says, in a voice identical to yours, that you need to get out of his house.