Those three are best friends. They have their respective secrets. And they're deliberately trying not to give themselves away. But neither Orihara or Shizuo had any secrets whatsoever. And that resulted in the complete opposite kind of personal relationship.
He may seem cold-blooded, but he's more human and his heart is more brittle than anyone else... so much so that if you filled it with human love or betrayal, it'd break easily... which is why I think he chose to avoid it all from the start: to love humanity. Do you understand? Not to accept it, not to face it, but he chose to avoid it.