I think the main reason he took the belt was just to mess with it. The reason he didn't give it BACK is kind of... the belt locks onto the first person who uses it, and once someone uses it it goes from blank black to have a little decal on it, which would give him away. But the main reason he didn't is, he feels trapped by the expectations placed on him by his family, and he feels really helpless. He sees the belt and the powers that come with it as a way to (literally) fight for himself, on his own terms? It's something that hasn't been planned out for him, so it's HIS and that's really important to him.

GOD YEAH... grape... we're expected to take this all very seriously but it's a show about fruit warriors. it's so good though!! it's not too far in yet so it'd be easy to catch up.