Is he rubbing his butt on the arm of the chair, or just sitting there?? If he's rubbing could be... just an itchy butt, worms, etc.

When he sits on it, is he higher up that you are? If he is, then its a dominance issue -- the dog who is up higher is the alpha dog. He's trying to challenge you. IF this is the case, don't give him attention simply pick him up and move him to the floor.

Otherwise..... he's just comfy!