Why did luffy and usopp fight?

Answer 1

I'm presuming your refering to the fight between Luffy and Usopp back on Franky's island WaterSeven/Aqua Lagoona.

Well they were fighting over whether they should leave their ship, Merry, behind as it had become broken beyond repair after their visit to the Sky Island.

Usopp wants to save and repair the Merry, but Luffy see's sense and knows it's safer to leave the Merry behind.

Usopp challenges Luffy to a dwell, which Zoro forsees and prevents the others from getting involved and stopping them.

Luffy, enevitably, wins.

Answer 2

I think the fight starts episode 236 :3 it's a really epic fight too.

Answer 3

The going Mary was beyond repair and luffy decided to scrap the Mary and usopp fought for the right to keep the Mary.