Historically they are reflexive forms, the suffix -s is a remnant of sik, an old reflexive pronoun, which shows up in Old Norse as the suffix -sk, e.g. Hann heitisk 'He is called'. In Old Norse it was used to make passive constructions (which doesn't really show up in Modern Norwegian), but Norwegian did inherit the vestigial middle voice, implying subject is very personally involved in the verbal action (e.g. Jeg synes) or reciprocity (e.g. Vi sees*).