It's a hot day, Sanji walks out of the galley holding two glasses of iced, colorful, and probably alcoholic liquid. He gives Robin her drink first, then rounds the ship all the way over to the lawn deck to give Nami hers. As he's handing the drink to Nami, Zoro speaks up because obviously, if someone's drinking he has to have a drink too too. Damn alcoholic. When Sanji tells him to go fuck himself it escalates.