Sirius didn't get the whole...hiding thing and not sharing what he felt. There wasn't much Sirius hid, and that...only really pertained to his brother. But that...that had even started to get...opened up a little. But anything to do with Moony or Prongs or Wormtail? No, that Sirius didn't hide, and he didn't understand why Remus hid it.
The look on Padfoot's face clearly spoke toward his view on the idea of it being 'nothing.' It was clearly something to Remus, which meant it wasn't nothing. And Sirius wanted to know. Remus had said he would be honest and such about things the way he had on the journal, though even with whatever was on the journals, Remus had been trying to hide it. Hiding, hiding, hiding.
"You're talking to the person you say doesn't spend enough time thinking," Sirius countered, "And I'd say you've probably spent too much time thinking about it, knowing you. What the bloody hell would make you think my feelings toward you are going to change? And...'want' - as if that's all there is? Is that what you think, what you fear?" It hurt his feelings a little bit to be accused of such, but even more so, Remus had to get it through his thick skull that he was being an idiot on this matter.