“There was the initial euphoria of finding himself alone at college, free of everything, completely on his fucking own, and with it an optimism that here among these thousands of young people he would find someone like him. That, alas, didn’t happen. The white kids looked at his black skin and his afro and treated him with inhuman cheeriness. The kids of color, upon hearing him speak and seeing him move his body, shook their heads. You’re not Dominican” (Diaz 49).
"Inhuman cheeriness" is the forced kindness that the white students exhibit towards Oscar, because they do not really accept him.
"You're not Dominican" shows that even the students of color have western ideals of what a Dominican person should and should not, influencing the deterioration of his identity more because he is stuck in a place of limbo between being too Dominican and not Dominican enough.
"An optimism that he would find someone like him" means that he was expecting to be accepted, to find kindred souls, so the realization that he will not fit in to white western ideals is even more hard for him.