The drama in manga (shonen or otherwise) is often pitched to the extreme, and while Eiichiro Oda's One Piece is no exception, the sincerity with which he approaches the big, dramatic confrontations makes sure they carry as much emotional impact as they do physical. Volume 35 has a great example, in an argument and fight between Luffy and Usopp that is so painfully emotional that it becomes hard to watch. Friendship is a key theme of shonen manga, and we've come to care about these characters, so seeing them come to blows over a disagreement is awful, and Oda makes us feel every bit of emotional and physical damage they do to each other.
The thing that makes this work so well is that neither character is in the wrong. Luffy had learned that their ship, the Merry Go, has been damaged beyond repair, and he made the tough decision to acquire a new ship, even if this means leaving their beloved vessel behind. But when Usopp finds out about this, he refuses to accept it and, fueled by frustration over recent events in which he was badly beaten while trying to stop some villains from stealing some money from the crew, he up and quits, challenging Luffy to a fight for ownership of the Merry Go:
As dramatic as this is, it's a painfully heartfelt scene, an example of how friends, due to being close enough to know each other really well, can cause so much more emotional damage than strangers. This isn't a petty squabble; it's a disagreement over the very soul of the Straw Hat Pirates, and Oda makes it feel like the punch in the gut that it is.
The actual physical fight is just as hard to watch; while Usopp doesn't stand a chance against the far more powerful Luffy, he doesn't hold anything back, using every trick at his disposal and demonstrating that he didn't make this challenge lightly. Here's the key moment of the fight:
Just look at Luffy's expression after he gets blown up; Oda manages to capture a combination of anger, sadness, and determination that perfectly conveys his mental state. He's going to win this fight, but he's definitely not happy about it. And that final panel, in which the water splattered onto the Merry Go makes it look like the ship is crying, could be corny, but it fits right in to the highly emotional scene, especially when seen alongside the panels that painfully flash back to earlier, happier times. This is operatically dramatic stuff, and a great example of what makes this series so great. I just hope they can all make up before I'm too emotionally devastated.