After losing Ace in the war on Marineford, Luffy fought against despair, with the help of Jimbei.
Finally, he managed to hold on to something, his desire to see his crewmates. But he had realized how weak he was in comparison to all the top-tier pirates and marines that awaited him on the New World, and so, he send a message to his friends.
He was postponing their travel for 2 years, so that they could train, and grow far stronger.
Two years later, Monkey D. Luffy has grown incredibly strong. Has gain control and understanding of his Haki, and now uses it in battle.
We've yet to see Luffy full potential, but little by little, he's showing us new moves, just a taste of what his full strenght is.
Monkey D. Luffy won't let anyone die again, he's grown far stronger than he was, just for that...