The Origins of Rool's Argument

Rool didn't just pluck these ideas out of the air, did he? He had a particular way of arriving at these conclusions. Not even Rool's could find anyone who'd held the Difference Principle. His idea is that quite often people disagree about justice because they're biased by their own interests. But how did he get to that position? I mean, what mode reasoning led him to that conclusion?