Mr.P contemplates Arnold's anger knows that he cant live in this dreadful reservation and advises him to leave or else he will be killed. He knows Arnold cant fight forever so its better for him to vacate so he can have a bright future and not a atrocious future.
Arnold's and Rowdy's Friendship has been pure and unvarying,but when Arnold brings up that he is moving to rear den rowdy loses and becomes enraged. Arnold tries to comfort him so he touches his arm and rowdy violently puches him in the face. from this Arnold feels crestfallen we know this because he says "my heart broke into 14 pieces".