"Really? You liked Gaius even back when you were lying to him and Sirius about being brothers?"
An interest in Gaius certainly cast Regulus' actions back then in new light. It made more sense for Regulus to try and break the two men up out of jealousy than the spite Regulus had always claimed motivated him. Even if Regulus' desire was unconscious at the time, Harry thought it was probably a harbinger of the man Regulus had become.