Ryuk is different from most Shinigami in that he does not want to simply pass time away in the Shinigami Realm but find something to relieve his boredom, and he is unaffected by the mocking he receives from other Shinigami for it. Ryuk regards you as "interesting," specifically due to your experiments and the loopholes you discovers regarding the Death Note's rules and instructions, occasionally discovering abilities of the notebook even Ryuk did not know existed.