Reason?
Very simple. Zhong Guoren treated them too well! almost at time, I felt the people were worshiping them: watch their most stupid soup operas, dress like what they were dressing, being fans of their childish actors or actress. Basically, Korean were spoiled in Zhong Guo. That is why they look down at us. Any other places in world were they being treated as such. NONE.
They must feel that Zhong Guozen is inferior to them, therefore, they hate Zhong Guozen!