I would say a child develops Narcissist traits as a coping mechanism to deal with a profound sense of shame which is caused by a deep sense of rejection caused by an emotionally unavailable or abusive parent.
OK, I understand this.
However, in my case, I don't believe this ever happened to my covert narc sibling. I suspect they were very emotionally needy as a child and what ever love, affection, attention, praise, encouragement, etc they got they felt it was never enough.
Maybe having to share their favoured parent must have made them feel like they had lost something i.e if it wasn't for sibling (me being the sibling) I'd have you all to myself. Why do we have to consider sibling ? For activities or holidays or treats or days out ? Why do I feel I'm only getting half ?
Did they think sibling isn't equal to me, IMO, so how come they get what I get ? Why isn't it all about me me me all. the. time ?
Oh, and why we're about it, maybe they thought their non-favoured parent can 'do one' as well.
Like a PP said, it's innate. They were born with it. It exists in their head.
I have a relative who was over indulged by one parent and could do no wrong as far as the indulging parent was concerned. Indulging parent was still worrying about and running after relative long into indulging parent's retirement years. It is for this reason that they have never been able to have an adult relationship because they just expect to be worshipped. When indulging parent died, they got nasty and chose to estrange themselves from the family.
My spouse has remarked of my covert narc sibling, 'I told you, she's (insert name of above relative).