Curate, connect, and discover
He tied up the bow himself because he saw that people put bows on pets and plushies they liked. He thought that maybe if he had a bow, soemone would find him cute and help him. Eventually he realized people saw him as nuisance and a monster no matter what he did, so he stopped trying to get help.
He still kept the bow on, to comfort himself by pretending that someone did love him.
He likes it when the prefect puts bows on him, it comforts that younger Grim, its a reminder that hes loved like he always wanted.