The only thing that Usopp had ever wanted to be was a pirate. He wanted to be like his father, a brave warrior that sailed the high seas and never backed down from anyone. A man; with honor, and integrity, and strength, and the drive to sail around the world without a care or worry.
Luffy had given him the opportunity to do just that.
And it was terrifying.
It wasn't that he didn't like being a pirate; in fact it was just the opposite. He loved being a member of the Straw Hat Pirates. The people Usopp traveled with now had become the family he thought he had lost. Usopp knew that if his dad could see him now, that he'd be proud.
That didn't change the fact that since Usopp had joined Luffy on his quest to become the Pirate King he had been hit in the face (more than once) with a four ton bat, nearly fallen out of the clouds, struck by lightning, been impaled (more than once) by a huge man-wolf out to kill him, and been attacked by zombies. And that wasn't even counting all of the horribly dangerous and irresponsible things that Luffy did for fun or all the weird things that were considered normal on the Grand Line.
But it seemed that the further the crew traveled the less things like that bothered Usopp. Heck, it was getting to the point where going to new places that wanted to kill, maim, or otherwise make his life miserable was exciting; if only because later they'd be kick ass stories. Better yet, they'd be kick ass true stories that he could tell Kaya when he got home.
Plus Usopp knew that his crew would always have his back, just like he would always have theirs (within reason, of course). If there was one thing that he had learned about becoming a true man, it was the importance of trust. Trust in himself, along with trust in his crew.
Because the Straw Hats were more than your everyday run of the mill pirates. They were a family, and a strong one at that. Luffy was more than his captain, more than the future Pirate King. He was the brother that Usopp never had. The same was true for Chopper, Zoro, and the rest (except Nami and Robin, they were sisters).
So yes, being a pirate on the Grand Line was scary; and yes, the government was terrifying; and Usopp had to admit that his captain was more than a little insane. But in the end it didn't matter. He was Usopp, King of Snipers, liar extraordinaire, brave warrior-in-training, traveler of the high seas. And while he didn't have over eight thousand followers (yet, he was working on it), his family more than made up for it.
AN: Usopp the Liar has the trait of trust? Ironic, perhaps. But very fitting, considering the speech he gave Robin on the sea train.