People that think they deserve something someone else has that they don't and throw a hissy fit about it.
One family based example I have of it is a number of years ago, me, my mom and my sister all got laptops.
My mom didn't take care of hers, and it got broke.
My sister didn't take care of hers, and it got broke.
I took good care of mine and it lasted for years before eventually wearing out from age.
During the time I was the only one with one, I overheard my sister telling my mom how unfair it was that I was the only one with a computer and that she should get mine. Her only reasoning behind it was that she thought that for some reason, she deserved having a computer more than I did, even with her long track record of not taking care of anything she owns.
There's never been any sibling rivalry between us, and we were adult age at the time, so not just kid being a whiny kid thing. So, not really sure what prompted this switch to flip that started her towards becoming a self entitled whiny brat.