What is the "luck" factor in the story "The Rocking-Horse Winner"?
Luck is, according to Paul's mother, is "what causes you to have money." She says, "If you're lucky, you will always get more money." So, this issue of luck becomes a fascination for Paul. He hears the house whispering, "There must be more money!" and thinks if he can just get it, the whispering will go away, and his mother will be happy and love him. Even on his deathbed, his last words to his mother are "I am lucky!"
It is an unfortunate, tragic tale of the dangers of seeking after materialism as a substitute for love and happiness, and in this story, luck was the name that materialism bore.