Natural selection works on every individual’s relative advantage compared with others; hence, gaining an absolute benefit is insufficient.  If individuals were satisfied with any absolute benefit, they might still face negative fitness consequences if they were doing less well than competing others.  It makes sense, therefore, to compare one's gains with those of others.  

Sarah F Brosnan and Frans B M de Waal – “Evolution of responses to (un)fairness”


the morality of fairness is neither ... basic nor ... straightforward – and it may very well be confined to the human species.  The fundamental problem is that in situations requiring fairness there is typically a complex interaction of the cooperative and competitive motives of multiple individuals.  Attempting to be fair means trying to achieve some kind of balance among all of these, and there are typically many possible ways of doing this based on many different criteria.  Humans thus enter into such complex situations prepared to invoke moral judgments about the “deservingness” of the individuals involved, including the self, but they are at the same time armed with more punitive moral attitudes such as resentment or indignation against unfair others.  In addition, they have still other moral attitudes that are not exactly punitive but nevertheless stern, in which they seek to hold interactive partners accountable for their actions by invoking interpersonal judgments of responsibility, obligation, commitment, trust, respect, duty, blame and guilt.  The morality of fairness is thus much more complicated than the morality of sympathy [helping in response to need].  Moreover, and perhaps not unrelated, its judgments typically carry with them some sense of responsibility or obligation: it is not just that I want to be fair to all concerned, but that one ought to be fair to all concerned.  In general, we may say that whereas sympathy is pure cooperation, fairness is a kind of cooperativization of competition in which individuals seek balanced solutions to the many and conflicting demands of multiple participants’ various motives.  

Michael Tomasello – “The Natural History of Human Morality”