Some decades ago, child development advisers, and followers not always qualified, emphasised the importance of praise in a child's upbringing. So we have excessive praised for standard developmental achievements, with the result that growing children believe their achievements are much better than they actually are.
At the same time, the needs of the child have been emphasised over the adults'. Children have grown up thinking they are marvellous and that their needs must be met. They have not learned how to empathise with others, only how to best them and overcome any opposition. Threfore, criticism in adolescence has been unbearable causing anger and resentment.
A narcissist has scored high enough on the psychological test that assesses this to be given the NPD label. However, twice as many American college students gave narcissistic responses in 2009 compared with those in 1982. Time for change in child rearing!