Their friendship being quite forced and unexplored is one of the biggest issues I have with this ship, truth be told. Would be a lot easier to believe Kairi is the girl Sora cares the most for if in KH1 they portrayed her as something all that unique and amazing.
For instance, part of why CloTi works compared to CloAe is that both relationships are developed, but CloAe has Zack holding it back and quite a strong 'mother' motif to it, breaking the romance, but it's not like you're left with a 'default'. CloTi is based and established in its own right with a childhood promise and them both doing their very best for each other.
SoKai on the other hand you barely get their friendship expanded upon in KH1. Hell it's more or less shot down by Tidus, Selphie and Wakka when they praise the very ground Riku walks on and are already planning RiKai's freaking wedding. Sora's never really good enough and the one who seems interested at all, even if for teasing manners, is Riku.
I'll give you the Paopu drawing bit in the Secret Place but seeing the non-romantic meaning the Paopu can also have and how the whole thing became a home-symbol for Sora, I'd sooner relate it to his insecurities about the journey than SoKai, friendship or not.
And then what? She vanishes and he has to save her. WONDERFUL basis for romance right there, wot? And I'm supposed to believe that after I spent most of my time on DI play-fighting TSW, that if any of them were to vanish that Sora wouldn't have gone after THEM?!
Oh and of course her Heart being in his vessel biz. Contact between Hearts is what got Sora to be basically Ven and for Riku to become Rikunort. While on a lesser scale, excuse me if I doubt that Kairi's Heart coming into contact with Sora's had NO impact on the kid :V Especially with how glorious the coincidental circumstances were, how as soon as Kairi's Heart was out of Sora's body it was "oh derp, Kairi, you'd get in my way, stay here and then on DI while I go chase Riku".
And then more than one bit where it's implied Sora's a freaking RIKAI SHIPPER. Excuse me if I don't think he'd go after Riku's girl.