I was thinking that maybe Sirius finally realized that Severus really did love Remus and that Severus's speech about how Remus didn't know what love was struck a cord with Sirius. That Sirius knew it, but never really admitted it to himself how Remus was. All of a sudden Severus speaks it aloud and Sirius knows that Severus may be acting crazy, but he's not stupid and that's why he's being pushed to his insane behavior.