Going to guess Zero is busy again, in other news, I'm trying to come up with a good goal for Virgil, and debating whether or not I want to inflict the Will of D. on him.
He was going along with being a Marine, but got drummed out. Perhaps he has a certain love of justice. Perhaps he was trying to pay back someone by becoming a Marine. If he is a D. then he's either an orphan who is trying to pay back the people who raised him by becoming a marine, or Garp put him up to going through the training.
"I couldn't be a Marine, so I'm gonna be a Pirate!" While somewhat shonen, sounds awfully childish.