It's frowned upon to be selfish.
Sally's mom gave her a bag of chocolates to share with her classmates.
Sally wants to eat chocolate.
Sally takes some of the chocolates for herself then shares the rest with the whole class.
Sally keeps the whole bag of chocolates for herself, eating it all by the end of the day.
Sally's classmates are very pleased with her and think she is very nice.
Sally's classmates think she is very selfish and don't like her much after that day.