"I'm not forbidding anything." Riku wasn't saything that Sora should pretend it never happened, Riku just didn't like to hear about it. Was it so wrong that he didn't like it? Grrr.
"You don't understand," Riku said irritably. There were no ghosts, no ex-es, in Riku's past that Sora had to deal with, be compared to, or compete with. Certainly none that could be called 'best friend.'
And it wasn't as if he even implied that Sora had to, or should choose between them! Riku thought himself quite patient on the subject of Kairi; never said anything about how much Sora talked about her, or called her, or anything.