He takes other jellis heads. Not out of spite or mean to harm. He cant really feel so much. Hes very animalistic and somehow is alive withouth a head but he doesnt like having no head so he decides its okay to "borrow" someone elses head. When he does the body of the head dies if enough time has passed but the head stays alive and santient (separate mind from the body) so this experience is usally very traumatizing for the victims.

The victims head (and mind) stays alive because the "new body" sustans it by connecting its jelly and providing nutrients. The body cant see or feel etc through the new head but is more of just a replacement. The head cant control the body either.