Reggie's smirk was perma-fixed on his face during this entire scene. Not only was it adorably hilarious that Luke and Julie were trying to pretend like they hadn't just been making out, but he was also just genuinely happy for his friends. They were good together.
When Alex suggested they leave, however, his expression turned to one of bewilderment. "What? Leave? We JUST got here!"
Alex responded in kind by smacking him on the arm and giving him a pointed look. "What? I don't know what you... OH." He chuckled softly and turned back to the other two. "Right. We can go. Leave you two aloooone."