Derek was being tested.
The entire time that he had been going down on Emily. it had been a painfully exquisite form of torture for him. For a while Derek had been carrying Emily over to the depths of pleasure with nothing but his fingers his mouth and his tongue.
He had been denying himself any form of release. His manhood full of blood, hard to the point of pain. Thrumming with arousal. But all the while, Derek had not even touched himself.
He was someone who was used to his every desire being fulfilled almost as soon as he had it. But from the moment that Derek had seen Emily come apart. His own desire had taken a back seat. His focus on seeing her in the throes of pleasure once more.
It was the most selfless that Derek had ever been, and he was beginning to see the allure of doing for others.