I imagine Sora wouldn't have even thought to make allowances for Riku's insecurities circa KH1, because from what we saw there, Riku's modus operandi was to mask his insecurities by being very cocky and confident of his skills. Which is doubly bad, because Sora isn't exactly so psychologically incisive (especially then -- he's quite a bit more so by the end of KH2) that he can see past the defense mechanism to the insecurity, and because it means that Riku's immediate response to 'I perceive that my friend is rejecting me' is to get even more distant himself.

Yeah: Riku clearly doesn't judge either of the two of them by his own impossible standard. Fortunately, by the end of KH2, Kairi and Sora have matured enough and figured enough out that they know to grab him and dig their heels in and hang on when he pulls his 'I'm not good enough for you, I'm going to pretend to be all aloof and run away' trick.

(This is all spinning around in my head because I"m working on the OT3 -- I've gone all meta-y inside my own head....)