Probably a combination of boredom and stress from being in confined space. Is your Valentini puffer eating?Offering some extra nice foods like raw squid, clam or shrimp might help. I trained mine to come up to the surface by tapping lightly on the floor just before feeding time. Within a week, s/he was up at the surface when I rang that dinner bell. Seems s/he would focus on anyone walking within range of the tank and less on an environment which doesn't have all-day access to foods.