Luffy was their captain. He could often be dim-witted, addle-brained, and immensely annoying. He usually ended up punching everyone sometimes because he felt like it, not knowing the most obvious things, and eating all the food. He could be reckless and stupid, but he was their captain because despite that, there was something more.
It didn't matter who you were, but if you needed help, Luffy would always be there. He had defied death so many times for his crew, even if it meant he would never reach his dream. If he was to be King of the Pirates, he would certainly be the best the world had ever known.
Zoro reflected to when he had been held captive at the marine base. He had just been a pirate hunter, sentenced to possibly die because he had saved a little girl. Luffy had no reason to save him, but he must have seen something in him, for he risked his life and defeated Captain Axe=Hand Morgan. He'd punched Helmeppo because he had lied to the swordsman and was planning on having him executed rather than fulfilling his end of the bargain they'd made.
Luffy risked his life for each of his crew mates over and over, going after them no mater what peril they were in. He never left anyone behind and never gave up. This was what made him a true captain.
So, the crew had often joked once in a while, pretending to be captain, but they all knew that Luffy was right for the job. Because despite his stupid rubber brain, he had a pure heart.
