Nishino wasn't in love with the easy classmate from the start, he just decided that of every girl in the class she looked lke the best one to him, it matched his idea of a highschool girlfriend and personal taste, he left her behind instantly and pim guy was unable to ever provoked him in the slightest.
As most people around him are wicked and rotten, and he knows it, he treats them as pebbles on the road (find me someone that gets angry at a stone) and is searching for someone he will really be able to enjoy his youth.
Rose is the only one that truly cares for him right now and all the other people see him at face value, his being a loner in the class for a long time even messed up with his common sense, add the fact he have the patience of a saint and is hard for him to interact normally.
Nishino should change school or take a sabbath year from highschool and rejoin with younger people that don't know him to have a chance to live as a normal young man.