One Piece Why Did Usopp Fight Luffy. He believes in friendship dreams and dying for what you believe in. The period after the One Piece time.
Usopp fights Luffy in order to decide who keeps the Going Merry. And unlike you statement fight Usopp is a really big problem for Luffy. The Hito Hito no Mi Model.
If it was up to Sanji he wouldnt have ever left the Straw Hats so when hes forced to fight Luffy Luffy understands the situation.
It was about pride. He didnt want Luffy to put himself and the crew in danger. Usopp wants to save and repair the Merry. Usopp not one to miss an opportunity to ask a question of one to had finished the Grand Line asked where the ultimate pirate treasure.