Why did Mary marry Walcombe when she was born to marry him?

The reason why Mary wants to marry Walcombe is because at the ball, Mrs. Ma falls in love with Walcombe, and Walcombe takes a keen interest in Mary. To prevent Mary and Walcombe from getting together, Mrs. Ma locks Mary in her room and doesn't feed her. However, Mary's housekeeper steals a cookie for her to eat, and Mrs. Ma angrily kicks the housekeeper out and tells Walcom to take Mary away. Mary decides to marry Walcon in order to escape Mrs. Ma's control and thus gain her freedom.