If it was two days ago, he would probably be confused on why he would feel this way. But after he admitted his unhealthy fixation over Haneul, he immediately understood the reason why.
He's jealous.
That's right—he was jealous of the closeness between Haneul and Robin. Just thinking about how they hugged so naturally after Haneul's name was called made Taeyang's jaw clench. And just now, Haneul had glanced back to silently cheer for Robin, not sparing so much as a glance in Taeyang's direction. That last part particularly stung.