When parents make their kids selfish.

 Sometimes parents unintentionally make kids selfish.

For example, you have a couple of kids. Among them, some are good in their study and some are not. To encourage the good ones, you always give them something extra. Like chocolate, good food or give more amount of food. You give this extra benefit without informing the other kids. Because you don't want them to cry for it or to get disheartened. Or sometimes, they know about it. 

Now the genius ones know that they are getting extra for their extra marks or effort on their study. So, they are the superior in the family. Hopefully, they will continue to do good in study or better.

When everyone grows up, you will see, the people who got extra in childhood are always expecting extra. No matter what if they get a chance, they are taking the best one, they don't share and sometimes they will hide things so that they won't have to share with others. Or we can say they became very selfish. Because this is how they get raised. You have made them like that.