Why are people not contented by what they have?

In a class of children I can see they have different personalities. A happy go lucky sort of child will always be happy. When he or she is older they will probably enjoy his or her career. A mover and shaker type of person might aim for complete success and not be happy however far up the ladder of success. A person might have a depressive nature and be miserable almost all the time. A smile doesn't cost anything and everyone could do that. A rich person might not be all that happy or contented. A poor person might be happier working at a job he loves and finds satisfying like a school care taker. Some young people have no idea which career they wish to follow.