You should care about other people.
John is in the front of a long line at a busy bakery well known for their delicious doughnuts.
John wants to buy some doughnuts.
John buys a couple of doughnuts, leaving the rest for the other customers.
John buys every single last doughnut, leaving none for all the other people who have been waiting in line.
The people in line behind John also get to enjoy some delicious doughnuts and the bakery gains more future repeat business.
The people in line behind him get upset and criticize John for being so selfish.