DISCLAIMER: I DO NOT OWN ONE PIECE. IT ALL BELONG TO ODA EIICHIRO
The captain of the Strawhats pirates was sitting in his favorite spot on the Thousand Sunny-go looking out to the ocean in his new ship.
The Strawhats was just leaving Water Seven after meeting Luffy Grandfather.
When Chopper the ship's doctor comes up to him and asks him why he didn't become a marine like his jiji. The rest of the crew was thinking the same thing.
There are plenty of reasons why Luffy does not want to be a marine.
One his grandfather is a marine.
"Luffy" said Grap his grandfather. "You are going to be a marine."
"Why?" said Luffy, Grap reply because I am a marine and I will train you to be a marine just like me. Luffy who was four years old at the time ask what does marine do? Grap said well we train, follow orders that are giving to us and capture pirate who run around the oceans think they can do whatever they want.
Luffy ask where will you be jiji.
Grap said training you to get stronger every day.
Now Luffy who is now already experience his jiji type of training? Luffy can recall the horrors of thinking he was going to die and he was only four going on five years old.
Luffy knew that if he decide to be a marine he would never escape his jiji training and while he loves his jiji he thinks might die one of these days. So Luffy thought really hard has much as a four year old can 'What should I do?'
Then he remembers his jiji saying that pirates can do whatever they want to on the ocean. Luffy turns to his grandfather and said "I want to be a pirate."
Two he meet a pirate name Shanks and decides he wants to be just like him.
Inside the Party Bar own by his friend Makino, Luffy who is now seven years old sits next to the pirate he admire.
"Shanks let me be part of your crew."
Shanks just laugh and said "Luffy you're a kid and a kid can't travel on a pirate ship. Especially one that can't swim."
Shanks think to himself, of course there's this problem where Grap the Hero might actually kill him if he put Luffy on his ship and as much as he value Luffy he also value he and his crew life.
After some time when some bandit comes into the bar for wine only to find it gone. Shanks offer his unopened bottle to the bandit leader instead he smashes the bottle and leave.
When Shanks starts to laugh Luffy get mad and turn around and eat a fruit he sees on the counter.
Suddenly Shanks is telling him to spit it out holding him upside down shaking him when his leg stretch and his face hit the floor.
"Luffy do you know what this mean."
"It means you can't swim ever for the rest of your life."
"What! Then how can I be a pirate"
Shanks shook his knowing that if he and his crew don't get out of town fast they really are dead.
"Ben gather crew and supplies we're leaving."
Shanks and his crew return back to party bar to say bye to Luffy only to find out Luffy was taken by the Bandits.
Shanks lose his arm while saving Luffy from drowning and from a sea-king from eating him.
While Shanks and his crew were leaving he leaves Luffy his hat and tell him
"When you become a great pirate you can return the hat back to me."
Luffy then goes on to tell him he will become a pirate, gather a crew strong enough to beat Shanks' and find the world's greatest treasure and no matter what, become the Pirate King.
Luffy know he can think of a lots of reasons why he don't want to be a marine, but the most important reason is that if he was a marine he would have never been able to gather up this crew that he have. That he probably would have never met them at all.
Luffy know that he made the right choices who knows what have happen to each one of his crew members if he never found them.
So Luffy turn back to Chopper and said "Being a pirate fun! Why would I want to be a marine? When I have everything I want right here."
Sorry for the grammar and mistakes I made in the story.