Am I the only one who really thinks that no, Sora ISN'T Happy-Go-Lucky until KH2, and when he was happy-go-lucky, it's established in-game to be a farce and he's breaking inside? :\
Most of KH1 was Sora worrying and I won't say angsting, but displaying sadness and longing and Donald and Goofy had to cheer him up. CoM? when in tarnation was he happy-go-lucky in CoM? The closest was Data-Sora but who knows how much rewriting that kid's been through...
I would be willing to say that you are correct that Sora isn't as happy-go-lucky as he seems. I would also like to argue that he was not so happy go lucky in Kingdom Hearts II. His expressions in KHI make it quite apparent that he has become sad throughout the failures of his travels. If anything I believe that most of his demeanor in the sequel is the confidence gained from saving the worlds in the first game. In Chain of Memories he started with outstanding amounts of confidence. Sora starts following a strange black cloak without Donald and Goofy. They eventually catch up to him but not until a bit later. The obvious character development that occurs in Chain is eventual loss of confidence, which results in a more serious nature, one that many of you seem to enjoy. When his memory is erased his confidence is regained.
Now that we find Sora awakening in Kingdom Hearts II, he is his happy and probably just as confident as he was in the beginning of Chain of Memories. I mean despite his small clothes he struts around showing his stuff. (You have to be cocky walking around in short shorts like his.) His past has made it easier to go through the second adventure without being fazed. But if one looks deeper into the psyche of our teenage hero, we can find the subtleties of the pain his adventure has placed on him and the changes that he has gone through. These can be expressed through a few situations where Kairi is mentioned.
[video=youtube;OUkHEC_DgBc]http://www.youtube.com/watch?v=OUkHEC_DgBc[/video]
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For example the line Sora speaks about Kari shows regret. It also shows that he is becoming a more conscious person, he questions his one actions and begins to analyze them.
Another scene is the infamous scene where Kari hugs Sora, but he shows a bit of shock. I believe that this not as most believe a sign that he does not like Kari, but much more of a realization that despite how powerful he is the confidence he had has subsided and the long hug that takes place is one of comfort for both parties as they begin to truly realize the severity of their actions toward one another as well how others are effected. This realization is completed when he reunites with Riku, this is why Sora becomes so emotional. He has reached a breaking point where the stress is far to great to hold the facade which Smile mentioned before.
I agree that Kingdom Hearts II had less character development for Sora, but I do not believe that is completely lacking. Optimism is a necessary key to becoming the type of hero that Sora has become. So many look to Sora when they are in dire need. Sora has to bottle up his emotions for the most part to gain their confidence, rather than being cold in the face of danger Sora chooses a different way to gain support and encourage those around him. Without the face that Sora wears, no one would put their trust in him the way they do.