I don't see Sora as "overly" happy and I happen to like Sora. Sora can certainly get upset or angry, or disappointed and any of a number of examples can be pointed out. But generally, he's naturally cheerful, and it's even harder for him not to be around a crew like Donald and Goofy.

Roxas, on the other hand, he definitely quite often seemed to be wearing a frown, or a neutral expression at best. But you could tell the resemblance to Sora most from those strikingly blue eyes. I wouldn't call myself a Roxas "fan," but I definitely found his story interesting, and do find his story-arc quite tragic, even if he does become an alter-ego of sorts of Sora. And I do love his tender leitmotif, which is an interesting contrast to the cheerful (but not annoyingly so) "Sora."