I guess it's because he believes Gyokuran has every thing he doesn't. He's blaming him I guess, for having the family that Shion himself almost had with his foster father and Kyaa. I love how he got another one of those cats, all chapter long he's all mellow and angry and then he just gets so happy when he meets "Kyaa" again. I wonder if the KK, is like Kyaa the 2nd or something?
Would be cool if everyone got these extended flashbacks, but Rin/Shion has gotten easily the most attention so far so I dunno.