Sanji always said that an "if" could make all the difference. Between a conditional and a true, no strings attached, statement. To Sanji, "if" could change the entire meaning of something.
The cook had been through his share of bad endings. From people saying that he'd never be a great chef, to the insults, even now, Sanji remembered each and every unpleasant memory, no matter how small of a disappointment it was.
He had decided early, when he was younger, that he'd grow on his mistakes. When the chefs told him he'd never be great, he made a promise to himself to rise above it, he'd become great, he'd find All Blue, he'd make everything he wanted, everything he dreamed a reality. There were no ifs in this statement.
To Usopp, "if" was a twist in a story. He could milk the story out so long that every person around him was hanging on his every word, predicting everything he'd say next, and in one small word he'd turn the entire thing around. Throwing in a catch, succeeding in keeping the attention going for a little longer.
"If" was Usopp's way of dreaming through words, of telling himself that he really was all these great things. No matter what doubts he had, he always believed that if he could tell it, he could do it, even if struggle was eminent.
Usopp's dream of being a brave warrior meant surpassing all of these things. Of becoming a man worthy of all the stories he told, of living the tales, going from famous liar, to famous legend.
When it came to the relationship they shared, there were no ifs. There weren't any: "Well, if only-"s or "Maybe if-"s when it came to how other people saw them. It was just Sanji and Usopp. The rest of the crew thought they fit together. Sanji listened when Usopp told the stories, believing that the sniper could accomplish every tall tale he told, and Usopp had faith in Sanji when he would dream out loud. Sanji always said that they could accomplish anything if they worked together to overcome all obstacles.
Usopp agreed. "If" really could make all the difference.