[RobaWho]

"It's not known what causes narcissistic personality disorder. As with other mental disorders, the cause is likely complex. Some evidence links the cause to a dysfunctional childhood, such as excessive pampering, extremely high expectations, abuse or neglect. Other evidence points to genetics or psychobiology — the connection between the brain and behavior and thinking."
"Narcissistic personality disorder is thought to be uncommon, affecting less than 1 percent of people in the United States. It affects more men than women. Narcissistic personality disorder often begins in early adulthood. Although some adolescents may seem to have traits of narcissism, this may simply be typical of the age and doesn't mean they'll go on to develop narcissistic personality disorder."

Although the cause of narcissistic personality disorder isn't known, researchers continue to learn more about the factors that may increase the risk of developing the condition. These risk factors may include:
An oversensitive temperament as a young child
Overindulgence and overvaluation by parents
Excessive admiration that is never balanced with realistic feedback
Unpredictable or unreliable caregiving from parents
Severe emotional abuse in childhood
Being praised for perceived exceptional looks or talents by adults
Learning manipulative behaviors from parents

[BykrBayb]

I’ve read that a major contributing factor is lack of bonding in early childhood. Sorry, I don’t have a link. I don’t remember where I read it. But it makes sense to me. A major component of narcissism is lack of concern for others. Concern for others is learned through bonding.

[delphirogatio]

It's not known what causes narcissistic personality disorder. As with other mental disorders, the cause is likely complex.

Here's the money quote for me, as it seems to help reconcile the fact that many people can come from similar circumstances (or even identical) and turn out completely different. This doesn't mean we should inquire about all the factors, but there is likely no magic bullet.

In my experience, the more pronounced the behavior, the more identifiable the cause, however.

[NEMDF]

From the people I have seen who seem to have this type of problem, they were brought up by parents who gave feedback along the line of making the child think he was special and superior, and the child believed it.