The dragon only serves those who is stronger than them, the only reason he was the was because of that slave color.
But he don't hate the prince for wanting to earn his service the right way.
The reason the dragon fought was because he wanted to fight the shikigami.
He lost and shikigami became qualified to be his master.
It might sound Like a 180. But originally the dragon was on neither side.
Finally!!! A not loli-jail-bait dragon.
Could've been shota jailbait since it was son of the dragon king but I'm already imagining all the Yaoi baiting and BL shenanigans this route gives us.
@RossiB2005 I honestly thinks its just because the trope has been played out too many times in the past few years.