Riku followed Sora, laughing too, because when Tsuna started, he bashed Gokudera's nose with his head, and Gokudera fell over onto his side, holding his face, and really, it was just so typical that Riku found it hilarious. And after ALL the crap that those two had put him and Sora though... suggesting that they were the ones dating, well, they deserved this.
"Come on, Sora," Riku threw his arm around Sora's shoulders. "You promised me a nap."