Okay, stick with me, we're almost there! After the secret ending of Kingdom Hearts 2 where it's revealed Sora left Destiny Islands to help King Mickey once more (because good is stupid), our Disney pals discover a long-forgotten note from Namine, which reads: "Their hurting will be mended when you return to end it." Eventually, they figure out that the note is revealing Sora's true purpose, which is to save everyone tied to his heart, including Roxas, Ventus, Axel, Xion, Terra, and Aqua. I kind of thought it was to save the whole galaxy but okay, I'm done asking questions.
Sora and Riku also go to meet with Mickey's old master, Yen Sid (i.e. the Fantasia guy), who instructs them to take the Keyblade master test so they can become true wielders and save the world from a new, encroaching evil. And by new I mean old, because apparently destroying both Ansem and Xemnas means Xehanort is going to come back fully formed, so the last four games meant literally nothing. FAN-FREAKING-TASTIC. But at the very, very end we find out that Yen Sid's also going to train Kairi to use the Keyblade, making her, Sora, and Riku a proper world-saving trio, so all is forgiven.
Explained in: Kingdom Hearts re:coded, Kingdom Hearts 3D: Dream Drop Distance