User blog comment:PirateKnight19/Luffy's Father sailed with Gol D. Roger/@comment-24850639-20170530234719

While you may not have any unique evidence, I've read this theory from a lot from people who do. While I'm not necessarily sold on Dragon ever being a pirate (I'm more willing to believe he could have been a marine at some point), I do definitely believe that he knew Roger well. Dragon appeared to at the very least be a young man at Roger's execution, so old enough to have possibly studied up on Roger's exploits and/or met the man himself. While I don't have solid proof of course, I'm almost positive that Dragon is aware of the history of the D. Clan, and understands full well who/what Roger is waiting for. I'll even take it a step further, and say that Dragon is aware that Luffy inherited Gol D. Roger's will, and knows his son is destined to have an incredible impact on changing the world. While its easy to point out all the times that he's seemed uninterested in hearing about his son's exploits, it's also easy to forget that he spoke of finally meeting his son as if it's the fulfillment of some kind of prophecy. I'm willing to bet that he acts the way he does because he doesn't want to interfere with his son's destiny by letting himself get too attached in any kind of way. Going back to me believing Dragon could have been a marine, it's just one of the several possible explanations floating in my head for Garp's over-insistence that his grandsons become Marines. We all know that Garp is not so selfish that he'd try to force that on them just because he was a marine himself, so I think it's something incredibly personal. Maybe something Dragon did as a Marine, knowing that his family will always have secret targets on their backs because of the D. Clan lineage, his hurt at seeing Roger be executed, etc.