How do people or why do they think everything in life is fair ?

[IsabellaVireauxLaurent]

basically anytime someone eg has an advantage or eg gets a promotion over someone, or what ever it may be, some will always say its not fair but how can life or any situation be truly fair when theres always context or other factors, plus who says whats fair, and to who etc

plus who would said what the common agreement is

Of course life isn't fair. The single biggest influence on how good it is is the accident of our birth. After that, the best we can hope for is fairness in specific contexts.
So it's reasonable to expect a promotion to be gained as the result of an impartial selection process between equally qualified candidates. But deluded to believe that everyone will have had a fair chance to become one of the equally qualified candidates.