A teacher cannot teach those who think that they know everything! In other words, if a learner is arrogant, simply you cannot teach this person!
"Who is to decide what virtue is?" Culture and sages decide what virtues are!
"If a horse does not want to drink water, then it is useless to force the horse to drink." A lot of learners in school do not want to learn but they learn simply they learn for their parents or other selfish reasons! Thus, I think that they are just like horses that do not drink.
"Why should it be the horse trainer to want the horse to drink? Why can't he leave it to the horse?"
The horse trainer are teachers and parents. Horses that do not want to drink are children who do not want to learn.