Narcissism is very adaptive, as it tends to grow on people and change over time in order to keep the person constantly feeling more and more special about themselves. No matter the age or gender of the group it is still coming up and taking over people causing them to become more narcissistic.
Narcissism is the ability to love yourself. This becomes a problem if this adoration is unrealistic, or prevents other social interactions. Narcissism does not really understand fat, thin, ugly, attractive, or any other real characteristics. The disorder adapts beyond logic most often. Narcissism is most dangerous when sudden changes occur. Rejection or mutilations can destroy a narcissists entire reality. This usually leads to other disorders that cause the complications most associated with Narcissism.
Narcissism is adaptive. To truly understand narcissism, and its potentially adaptive benefits
as well as detriments across the lifespan, requires multiple levels of analysis. Narcissism is clearly a multifaceted construct, and some components may be helpful for certain age groups and professions. I'm happy to see such integration has already begun.
You could treat yourself as the most important being in the Universe without necessarily being a narcissist. Part of narcissism is requiring that others recognize you as so important. But if you're really so important and really the best then why would you care? Narcissists secretly have an inferiority complex they are compensating for.
Narcissism is not adaptive. A person either has it or they don't. It is not a trait that can easily change throughout life. Unless a person has substance problem, narcissism tends to remain constant throughout one's adult life. In the growing phase of childhood it is barely adaptable, and especially not in adulthood.
This question is a bit confusing, so I'll assume what's being asked is whether or not narcissism is something that can be changed or cured. I personally believe that narcissism stems from early upbringing, and that it gets very ingrained into one's personality, to the point where he or she can't change. People who are narcissistic believe that the world revolves around them, and they generally aren't good at reading others' emotions, or even caring about others. This would be a very difficult, and likely impossible, personality trait to change.