"Oh, I'm not human? OK. This is my destiny."

once he realized that he was the core switch did his emotions as a human being deplinish, I sorta knew he was going to be the switch but i never thought it be like this. It was a good episode real good and had me crying like a baby...almost but i did shed tears

As Kengo said, when he realized what he was and what his purpose is, he acted on it so it wouldn't be any harder than it already was. That envelope he had in his room was a sign that he was still Kengo, but in the face of his friends, he had to play the role of "a sample". But because of Gentarou and Yuuki, he not only came to terms that he wasn't just a sample, but he was a friend of the Kamen Rider Club who he had such fond memories of for the past year.

The other reason why he probably accepted his destiny is not only because it was "hardwired" into his existence, but if he were to leave, Gamou wouldn't be able to succeed, and the Presenters would put an end to him themselves. The entire time, from his purpose to his inner turmoil, he was thinking of the safety and the wellbeing of his friends that he had to play the role of the emotionless sample.

Also, have you heard the rumors of other Riders? There is going to be a female Rider who uses the Peach EnergyvLock Seed. Zangetsu also gets an upgrade with the Melon Energy Lock Seed and a new type of driver that looks like a juicer. There will also be a cherry rider as well.