Sometimes, Loras needed a little perspective. He could be immature and downright selfish, but he was still a knight, and that at least gave him chivalry and a spark of selflessness where he was otherwise young and highly self centered.
But when Obi-Wan said that, Loras felt a sharp pang of guilt for whining about Renly, because at least...at least Renly had returned. He didn't know what to say to Obi-Wan to make it better, so he said nothing and only took Obi's fidgety hand and brought the palm to his own lips to stop him. All he could be was there for Obi-Wan, just like his friend was here for him now.
"I'll tell him the truth," he promised. He couldn't expect Obi-Wan to pretend for him.