How often do you hear or say those words! I hear them a lot and as a kid I used them a lot and as I reflect back on those times I can simplify the whole thing down to this.
Someone else got what I wanted before I did or some variation of that.
As a kid it's the things like who goes first in a game or gets to sit in the front seat of the car or gets to be the one to do something with mom or dad. I'm sure that brings back a few memories.
I didn't like it one bit and I made it known, using those famous words, that's not fair. I was resentful, jealous at times, vindictive and all those other things we do as kids (and sometimes as adults) when that happens.
I didn't realize at the time, having that reaction actually put me further away from getting what it was I wanted. In effect I lost out twice. Had my reaction been just a quick disappointment, then I would have got what I wanted faster even against the odds. Of course that doesn't change the facts that I still lost out on whatever I wanted, someone else still got it before I did.
We don't realize at the time that there will always be other moments, more things that we want. It's just that we have very narrow vision in the moment, when a broader vision is needed.
When things have settled a bit, parents can remind their kids when they were first in the past and how they will be first again, or get what they want again even bringing stories from their own past to demonstrate that.
As adults, it will be helpful to keep that in mind, at work with promotions and pay raises and in the current political, tax and other things going on around the world.
How did you react when you said these words, it's just not fair? How did it help or hurt you? Are you still acting that way today? As adults we may have changed those words to "that's not right"