Discontentment in Children

I have a child, who shall remain nameless, who is never happy.
Someone is always getting to do more than them.
Someone always has the seat this child wants.
"Why did you give him soup, you NEVER make soup for me."
"I'm hungry."
"Can I have candy?"

ALL DAY LONG.
I need help.
Do you have any ideas?
I have tried to talk to this child and explain that there will always be someone out there who has more, who gets to do more, etc.

I know what my Mom would say "Amy, she's just like you."
And I know she is, but I don't know what to do about it.
It's a hard way to live your life.
I've had to learn my contentment lesson the hard way, I'd like to ease her into it a little better.
Whoops, I said "her."
But I guess that is part of the problem I don't understand.
She is the only girl in our family for crying out loud.
She has her own room.
She always gets special girl privileges....
I looked at a newborn dress yesterday and said "Chloe, look at this for the baby!"
She gave me the evil eye because I wasn't looking for her!
SOMEONE HELP ME PLEASE!!!!!!!
Any ideas would be appreciated.