Arrested Development, Childhood Stages

Codependent people who also have Narcissism issues can be stuck in the childhood stage before one learns that the world does NOT actually revolve around one's self. If they have not matured past this stage for whatever reason, then they can't "know better", much more than a child at that development stage would.

So if you are a person in their personal life, they probably see you as a person who "should be" giving to them, supporting them, praising them, and helping them on a ONE WAY basis, the same as a child expects all of that from the adults in their life.

The child expects their parent to come to their school concert and watch them, clap for them, and give them encouragement and support, which is normal and healthy, and if the parents DON'T do these things, then the child will probably experience unpleasant or even scary feelings, and may also feel angry toward their parent.
BUT~ the child does not think about supporting, encouraging, or helping their PARENT with his or her activities, tasks, and goals in the same way.
Part of growing up is learning about being supportive, but "MUTUAL SUPPORT" that one would find between PEERS of the same age is not and should not be expected in a Parent/child relationship.

The Parent is expected to help and support the child a lot more than the other way around in a healthy functioning family. It's mostly ONE WAY.
Over time, the parent guides the child about being supportive of others, both by teaching and by MODELING the behavior. (Timmy learns about gift-giving, and why we give gifts, by his parents' behavior toward him and toward one another, and also from being directly shown and taught about it, like talking about what present he's going to give his sister for her birthday, and helping to pick out a card and gift for his Mom or Dad.)

Think about what Timmy is being "taught" if his parents behave otherwise.
If Dad goes golfing on his Sister's birthday and misses the party, or makes fun of women while picking a Valentine's Day card with Timmy for Timmy's mother, or if his Mom constantly criticizes, corrects, and puts down his Sister and his Dad, or if his Dad doesn't show up for Timmy's School Play after promising he would be there.
What is Timmy being taught when his birthday party is a huge celebration, but his Sister is given a minimal acknowledgment every year for hers. Or, vice versa.
Or if one parent gets big acknowlegement and recognition for various things, big celebrations for birthdays and Holidays, but the other does not. 

So if a child gets stuck for whatever reason in this childhood developmental stage and carries it into physical adulthood, where he or she sees others as people who should be supportive, helpful, and praising toward them, (and often "let them" be "The Boss", like they're still playing pretend games of "Soldiers", "Cowboys", "Rock Star", "Office", or "Tea Party"), but NOT THE OTHER WAY AROUND, then they are probably showing Narcissistic behaviors and traits.

YOU have to help ME, YOU should be supportive and understanding of ME, YOU should tolerate MY behaviors, YOU should PRAISE ME and give ME recognition and acknowledgement, YOU should LISTEN TO ME, YOU should treat me with RESPECT, YOU should come when I call you...

I don't WANT to do them, and I don't feel like you're important enough to deserve them, anyway.
You are here FOR ME, I'm not here "FOR YOU".

And I get to be The Boss. Because this is MY game, not yours.

I'm the one who is important, not you, so I get to be the boss over you, and I get all the support, praise, sympathy, attention, recognition, and rewards, both from other people and from you. You don't get any, because you're not an important person. I am.

If you played "Lone Ranger" with them, they would probably think that you SEE THEM as "The Lone Ranger", because he's the BOSS and the "star" of the show.
You can be Tonto if THEY ALLOW you to, he's a sidekick but he's pretty cool... or they might tell you that you can't be Tonto, that you have to be the random "maiden in distress" or "scared shopkeeper under attack".

Childhood Developmental Delay.
They're still back there, and they may not know it.
One can not be more mature than one is.