That said, we _can_ approach the phenomenon of narcissism structurally. Freud uses an energetic model based on thermodynamics: each of us has some degree of (psychological) energy, and we invest that energy in various ways. One of those ways is in fantasy, as Freud explained in his classic "little entrepreneur" model in _The Interpretation of Dreams_. The little entrepreneur, who has the capital (energy), invests in specific companies (fantasies) hoping to receive a return (satisfaction). This is the basic gist of what's known as Freud's "economic model".
Narcissism is the extent to which the little entrepreneur invests in himself. Immediately we can see that some narcissism isn't bad -- you need to invest in yourself! The problem is only when there's too much, or more specifically, when energy is _not_ invested into the world (or, more specifically, into others) because too much is invested in oneself.
Freud dealt with the seeming "paradox" of grandiosity and depression in his paper "Mourning and Melancholia". The simple overview is that you have a little voice in your head watching over your "self", formed by early authority figures, that says "no" (footnote 1). When you have a lot of energy invested in yourself but it's saying "no", then you feel depressed, because all of your fantasies are rejected. Sometimes you can suspend the voice for a while and say "yes" to everything, which results in a burst of energy suddenly available for use. This is called mania. The manic-depressive is caught in a back-and-forth oscillation of the voice that says "no" and the suspension of that voice.
In a general sense, if you have a lot of energy invested in yourself, you will feel grandiose when you can turn off the "no" and depressed otherwise. If you can figure out how to turn off the "no" forever, then you can feel grandiose all the time. If you can't turn off the "no", then you'll feel depressed, and you might go to therapy hoping that they will give you permission to turn off the "no".
And this isn't a bad thing: the therapist gives you permission to turn off the "no", suddenly the energy is available for use, and maybe you get through to the other side, when you realize that the promises of satisfaction through the self alone feel somehow empty in comparison with investing your energy in the world, in other people. The transformation that must occur is the development of the capacity to _love_. And narcissism is, to an extent, a step along the way (footnote 2).
There is a distinct phenomenon that "science" also seems to categorize as narcissism, where you might feel like you're the master of the rules of society, and that you have the right to control other people, do whatever they want, break the rules, etc. This phenomenon, which some might call "sociopathy" but as a technical term we can call "perversion", doesn't always come with a high level of narcissism, but can, and when it does, you end up with the sorts of villainous characters like Donald Trump etc etc. They wont seek "treatment" because why would they? They're already the masters. The social structure required to help them is really to incentivize them to act such that their personal interests align with the greater good, rather than creating an antagonism, because otherwise the greater good will lose. Another solution is to make spaces where they can act out their desires without harming others. On a personal level, though, it's pretty hard, but thankfully this sort of individual is quite rare, and oftentimes their actions lead to their own downfall.
1: not using the term until the end here, but the voice "above the self [I]" that says "no" is the "superego".
2: working off the Lacanian idea here that analysis proceeds through a stage of "maximal narcissistic projection" (Sem 1), after which the analyst can intervene from the position of the "Ideal-I" and basically allow the analysand to reorient their position wrt fantasy.