Title: Pain, Sweet Pain
Yuma is a sub. He wasn't always a sub, though. Back in high school, he actually tested as someone without a dynamic. No big deal. That's better. Yuma has seen what chaos dynamics can do to people, and he was just fine not having one. His friend Mao, however, manifested as a dom as a child, which is exceedingly rare and indicates just how powerful his dom tendencies are. Despite their differences, the two grow close, so close that Mao feels an innate pull toward Yuma. No matter how desperately he wants to deny it, Mao wants to dominate Yuma, but he does his best to avoid it for the sake of their friendship.
Graduation comes along, and the two are talking about their futures. Mao, though, is distracted. His longing to dominate Yuma is coming to a head, especially since this is his last chance to be so close to Yuma. Unaware, Yuma continues talking and laughing, enjoying the final hours with Mao, when Mao suddenly grabs and bites him. That single moment caused Yuma to suddenly manifest as a sub. Of course, it would have happened regardless, but Mao's bite triggered it to occur earlier than it would have.
Wracked with guilt, Mao distances himself from Yuma. On the other hand, Yuma spends the rest of his time chasing that bite. It's impossible for him to stay with any dom partner for very long because they're unable to give him that same satisfying bite Mao did when he awakened as a sub. In order to find that sweet pain, he goes through a matchmaking service with his doctor. To his surprise, he's matched with the person who introduced him to the pleasure of pain to begin with: Mao.