"a pirate cannot leave the crew without their captain's permission, so believe in Luffy." Usopp tells Robin, having already left the crew over his own stubbornness, knowing that Luffy will come through for Robin in a way Usopp thinks Luffy no longer can for himself ("he won't come to save me" he tells Franky)
"I have nakama who aren't that strong, but I still wish for them to be by my side. so I came up with this way to fight, so that I never have to lose anyone. so that no one ever has to leave somewhere far away." Luffy tells Blueno, having already lost Usopp due to reasons unrelated to strength and power, knowing he was too late in learning how to properly be a captain ("it's heavy" he tells Zoro)
but then Luffy yells at Franky that he hadn't forgiven him for what he did to Usopp, not knowing that Usopp is standing right next to him. and Usopp leverages his own life, as himself, using his own face, to motivate Luffy into standing back up against Lucci.
they were best friends and they both knew they hurt the other and they both regretted it so much and it was tearing them apart inside. they want to take it back but they can't they can't.
and Zoro. Zoro is so integral to this whole conflict. watching over them every step of the way. not allowing either of them to brush this under the rug. making sure Luffy shoulders the consequences of his decisions as captain. making sure Usopp takes responsibility for the choice that he made to leave. knowing that they needed to resolve this properly in order for them to truly move on, threatening that if they didn't then they'd lose him. hammering it home to Luffy what he needs to do because he knows Luffy needs to hear it. trusting in Usopp that he'd do the right thing because he know Usopp needs to take the initiative.
and finally, finally, being so happy and relieved when they resolve it the right way. the wide, proud grin and Usopp's name muttered under his breath as Usopp apologizes. the free laughter and telling Luffy he's the idiot as Luffy pulls Usopp back on board. Luffy and Usopp are back together, he hasn't lost either of them, and everything will be alright.