The laughing scene is pretty misunderstood.
It seems like most people think Tidus and Yuna are supposed to laugh "for real" and the voice-actors miserably fail, but the truth is that they are just doing it to make themselves feel slightly better.
That's why it sounds so forced - they aren't in the right mood to begin with.

Besides, if you listen to Tidus and Yuna right AFTER the laughing scene itself, they suddenly sound natural - because they start laughing for real at that point.