This is a fluffy moment, set after Zoro and Saga's fight during the fifth One Piece Movie. For those who want angst, check out my new series 'Breathtaking, in so many ways' :)
"He abandoned ship without permission, appeared before your eyes siding with the marines-your enemy, attacked your crew mates, and disappeared again without any explanation," Saga stated disbelief lacing every single one of his words. "And you still tried to fight me to save him."
Having been dismissed by Chopper, Luffy didn't bother to inspect his bandages and stretched his limbs instead. Once satisfied, he picked himself up, standing before Zoro's old friend.
"I'm his captain," Luffy grinned. "It's what I do."