People with garden-variety narcissism usually want others to be super-understanding, supportive, sympathetic, considerate, accommodating, and caring toward THEM.
But will actually get annoyed, angry, even enraged, and often disappear when it's time to give the other person support, help, understanding, accommodation, sympathy, or care.
It's not just that it's "one way", it's "ONE WAY or the HIGHWAY".

They want to be free to say and do anything they feel like, including treating others badly, and be given "understanding" for the way they behave, as if that's synonymous with being "nice".
But those who GIVE them "support", "understanding", "respect", "courtesy", or "care" are NOT supposed to expect it FROM THEM, apparently, for anything.

In other words, you're supposed to be "understanding" of them not paying you back a 10,000.00 loan, even if you can't pay some of your own bills because of it. But if you need a ride or some kind of help, you "brought your problems on yourself" and are some kind of "loser" or "bad" person. (Not in reality~ just according to a Narcissist).

You're supposed to be "understanding" of them insulting you, calling you names, threatening you, rejecting you, treating you horribly, and gossiping about you;
But if you even display any kind of "annoyance" or God forbid, anger or sadness toward them, you're a "BAD PERSON..."

You're supposed to be "understanding" of them looking at, flirting with, or even cheating with others...
but of course if you even speak to a member of the opposite sex who you've known for years, you're a "Bad Person".

You're supposed to cater to their every need, want, desire, and whim, because you care about them.... it shows you care about them...
But any needs, wants, desires or whims that you have are considered by them to be "burdensome" or "unnecessary" or "silly".

Everything is "one way". You GIVE , they TAKE.
They make requests and demands, you DON'T.
You fulfill THEIR requests and demands, they don't fulfill yours.
THEIR schedule is "important", yours is not...
THEIR friends and family are "important", yours are not...
THEIR financial well-being is "important", yours is not...
THEIR career and future is "important", yours is not...
THEIR emotional well-being is "important", yours is not...
THEY have 'natural' authority, privilege, and entitlement, you do not...

Narcissists live like LITTLE KIDS who set up CLUBHOUSES, trying to make their own little Hierarchy.
"No, JOHN is the leader! Because! Because he's the tallest!
And I get to be the assistant leader!
No, Jessie, you don't get to be anything, you have to do what we tell you, or you can't be part of the club. You have to bring the snacks."

This is NOT HEALTHY, this is NOT the way it's EVER "supposed to be", not in a family, partnership, or community.