Sirius looked like a child that had been scolded and put into the naughty corner. While he realized that he would have to address the issue of "Abby" with Regulus at some point...that didn’t mean that he wanted to. Sirius didn’t want to look at the smug expression on Regulus’ perfectly sculpted little face as Sirius gave him some "thank you for tending to my child while I was locked up" speech.