The app for independent voices
I probably should have said 'often' rather than 'usually'. It's also often posited that being spoiled can lead to narcissism and there there may be an underlying genetic disposition to it.
As far as being "social constructionist," that would only apply if your contention is that narcissism is an immutable characteristic, at least in the current academic sense of the term. "Social constructionism is a theory of knowledge that holds that characteristics typically thought to be immutable and solely biological—such as gender, race, class, ability, and sexuality—are products of human definition and interpretation shaped by cultural and historical contexts (Subramaniam 2010)."
Psychology has been largely captured by such postmodernist nonsense so it's always wise to be wary of that bias, but you can't just ignore it all. Here is an article that, in my experience and opinion, best sums up what narcissism actually is although it doesn't focus on causal factors, which can only really be guessed at--pretty much like most things in psychology.