Why did Jack leave the assembly and run away into the forest alone?

The boys gather on the beach to discuss what the hunters saw. Ralph tells Piggy about the creature on the mountain, which he describes as a beast with teeth and big black eyes. Piggy is skeptical. Jack assures the group that his hunters can defeat the beast, but Ralph dismisses Jack's group as no more than boys with sticks. Jack tells the other boys that the beast is a hunter, and he informs them that Ralph thinks that the boys are cowards. He continues his attack on Ralph, claiming that Ralph is no proper chief, for he is a coward himself. Jack asks the boys if they want Ralph to be fired as chief. When nobody agrees with him, Jack runs off in tears. He asserts that he is no longer going to be part of Ralph's lot. Jack leaves the group on the beach.