It’s because it’s where he grew up! He’s from an old family of dragons who were guardians to a royal family. Thus were gifted a large estate. Since the royal family dissipated, the guardians found themselves without a purpose, eventually breaking apart themselves. While Kentaro’s surviving brothers moved out to either pursue their passions or start their own families, Kentaro stayed behind in their mansion. He gets to live there for free, after all. That, and he feels a sort of responsibility to his ancestors to preserve the estate they all fought so hard for.