Princess Serena was tired of being taken for granted by her boyfriend. She felt that she had to stand up for herself and that is what she did. The mistress chose to trample him and to force him to respect her. So she trampled him after taking him by surprise and stomping on him body. She laughed at him and she enjoyed how much in pain he was after what she did. He knew better than to take her for granted after what she did to him.