Why does Woody's father force him to kill his beloved mare in the novel, West With Giraffes?

Woody's father forces him to kill his beloved horse to make a man out of him, but instead it causes Woody to reject his childhood upbringing as false. Woody loved his Mare more than anyone except his mother, and the moment he shoots her marks the turning point when he realizes that he must venture into the world alone and make a life for himself.