He's always played the part of a naive, oblivious child, pretending that he dosen't quite understand whats going on. But if you think about it, it's a ridiculous concept that he could could actually be so happy and optimistic after everything he went through.
Everyone spent all their time on Riku, all of their empathy and comfort went to him. But that wasn't very fair was it? He made a choice, he had options, he wasn't forced into what he did. Sora didn't get to make any decisions, he could either go and try to save his friends or he could leave them only to wallow in his own self pity. After the first year he really considered the second option, just forgetting about Riku and staying back on the islands where Kairi was sure to despise him. In the end though Sora made the right choice, he subjected himself to more pain and suffering, over and over again he traveled the worlds fighting everyone Else's battles because if he didn't what would everyone think of him? What would his parents think? if they were even still around. What would Goofy and Donald think? if they were really as loyal as they claimed to be. And what would Kairi think? If he even cared what Kairi thought at this point.
So after taking all of this into consideration, it's rather naive, and even oblivious to think that Sora was really as happy as he let everyone believe.
So when Riku found Sora in the bathroom, after twenty minutes or so of searching, he shouldn't have been shocked by the sight of Sora's small, lithe, body propped against the bathtub with his arms behind him dripping the contents of his radial artery onto to the smooth white porclein below. Nobody should have been shocked.