It's wrong to cheat in games.
Marc is playing Monopoly with his sister Louise, and she has bought all the best properties so far.
Marc wants to win the game of Monopoly.
Marc plays the game without cheating, and he still manages to buy some really good properties.
Marc moves the dice to get a better role when Louise isn't looking, allowing him to buy a better property.
Marc feels good about how he plays the game and it's very exciting when he lands on a good property.
Marc feels guilty about doing something so shady and even if he wins the game he will feel badly about the whole experience.