Well, he was in his own home, un-threatened, with Sora, and in a relationship that felt pretty stable at the moment. Why shouldn't he be in a good mood?
Riku felt bad though, when Roxas said he planned on going home and wished that he could do something to help. But he didn't feel right about hugging the little monster. Sora would have to do it instead.
"I... I'm taking Sora away, for a few days," Riku confessed, "And then we've got a date for the New Year. But I'll being him down to you after that." He paused for a moment. "Kairi is down south," he mentioned, "And I know Hayner went home." So it wasn't like Roxas would be alone.