^ This sorta furthers the idea that people just don't
get it.
I mean, you have the people on the one side who don't get it because they say, "Oh, JAT is a terrible actor, the voice/laugh is annoying and jarring and what even was it?" And it's like, no, Tidus is
forcing laughter... that's what forced laughter sounds like.
And if it's not that, it's flat out "the whole scene is annoying and cringe-worthy" I don't think is giving it enough credit. And hey, you're entitled to your opinion, not trying to tell you you're wrong
but you are 
but how much thought did you give to it? Are you looking at the laugh itself as a single act/dialogue?
In it's entirety, the scene is perfect (imo). It's starts with Yuna, excited to show Tidus that she's been practicing her whistling, and though he's happy for her, he's just had this whole big Sin=Father load dumped on him from Auron. So he's got some shit on his mind. His tone, uncertain. She picks up on it. It totally subverts their natural personalities. He's the happy-go-lucky flirty wanderer; she's the thoughtful and quiet summoner with a duty.
"Wanna scream?" She wants to cheer him up.
"Nnngh...." Has always been the epitome of JAT's perfect acting; such a natural, nuanced grunt that people make everyday and it sounds so seamless.
And then Yuna tells him how it's always been for her, and the whole characterization of her comes out in one perfect line
"I've learned to practice smiling, when I'm feeling sad, you know." And it just twists the knife in your chest because you can see the heartache and strife she's had to live through.
Tidus gets it too. But she doesn't want to leave it there. So instead, she's going to
teach him. And it's awkward. But it's fun.
"Next, try laughing out loud." And
he does! He's trying to put on a brave face for her when it's been the other way around. He's going to show her that he can buck up and keep smiling when he's feeling down.
And he's doing it. He's laughing so hard that the others are looking at him like he's got Sin-disease again. Yuna sees it; she's embarrassed him. Not what she set out to do.
But he keeps laughing. So she joins in. And before she does, as she takes in air, they share a look that asks
"Are we really doing this?" and his answer is
"In front of everyone."
And then they laugh. And it's still forced but soon they're roiling in real laughter, trying to compose themselves. And she
thanks him. Because she knows the journey's going to be hard but maybe it'll be a little easier with him around to keep her laughing.
The dialogue, the tone, the actions between these two characters and their relationship is possibly the most natural I've seen in an FF game. Because Tidus is a big shot around the girls back home but he sees someone he kinda really likes and he might not be good enough for her, so it makes him awkward. Because after the whole touching moment and with everyone watching, he just brushes her arm. Subtle but physical. And even though they knew everyone was nearby, when they turn around there's still a moment of shock because they'd forgtten; they were in their own little world.
So it's always astonished me, to say the least, when people mention that it's annoying, out of character, bad acting, etc, because it's probably one of the more real moments for these two as they're developing their relationship this early on.
(but it's just not as pivotal).
<3