First let’s put some time to our perspectives: Monkey D. Dragon was born 55 years ago. God Valley Incident took place on the island God Valley 38 years ago. Thus, Dragon was 17 years old when this incident took place.
For unknown reasons, the world nobles and some of their slaves were present at God Valley during this incident. In order to protect them from Rocks D. Xebec and his crew, current Marine vice-admiral Monkey D. Garp had to form a temporary alliance with Gol D. Roger, captain of the Roger Pirates.
Now imagine this. A young 17 years old fresh recruitment of Navy, working under his father as his subordinate.
The guy who faithfully embraces the idol of a father that Garp is, and the morals of Marine or should I say “the outward face of World Government” who has always said to thrive for justice and fight against who oppose it.
The 17 year old Dragon, then comes to God Valley.
How much would the ideals of someone looking to be a “Marine” would change if someone like Dragon would see the cruelty and inhumanity of the Celestial Dragon’s first hand?
I for one, can’t imagine a more convenient plot to explain as to why Dragon directly opposes the Celestial Dragons. I can’t think of a better backstory as to why he became who he is today.
*Theory by bibek070