The title is parasite because it is a song about you living inside me. Don't you think envy rots if left unchecked? Envy left unchecked for many years is hard to deal with. I think it is a love song because it is made to be spoken in the second person, but the perspective is pretty crazy, isn't it? I would say that's a good thing, though. Well, one thing I can say is that after spending so many years together, you lose track of the boundary between you and your partner, or perhaps you come to live in such an illusion, or something like that.