People love telling Ussop that he talks too much and thinks too little.
It's funny, really, because it's the exact opposite. He thinks too much. Talking is just a way to stop thinking.
Ussop swings his arms in a grand gesture. "And the great Captain Ussop stared down the Marine Commander without a flinch. 'Scum like you could never defeat me!' The mighty captain would die before he would surrender!" Chopper and Luffy are hanging onto his every word, and even Nami is hiding a smile behind the newspaper she's pretending to read. Ussop dives into the death match between the two foes, the Marine getting nastier and nastier with each near miss and low blow ('Why didn't my deadly poison stop you in your tracks? Arrgh!') while Captain Ussop meets all challenges and surpasses them with just enough difficulty to make it interesting.
He gets more and more wrapped up in his tale, an excellent story if he says so himself. The best ones always come on his worst days. Like this morning, when he lay in his hammock, his thoughts hissing at him that he was a failure. He wasn't brave or strong or smart, he would never be those things. His entire life he'd spent pretending to be something he wasn't, and sooner or later it would all come crashing down on him. Those thoughts hurt especially now, when he actually has friends. He's so afraid of losing them.
On days like this, he turns to the one thing he knows he's good at. The adventures of Captain Ussop are just a story, but they still give him a rush of confidence. Pretending that he's a brave hero, someone who's never known fear.
He's a liar, everybody knows that. It's only expected that he'd lie to himself.