Where do you suppose we ever got the notion that life would be fair, or even that it should be? No one thinks it is but we all think it should be. Why is that?
Discussion
Because our species survives better in cooperatuve groups. The desire for fairness, as well as our ideas of morals developed to help the individual fit better in groups and keep groups as benifical to all involved as possible.
If we were solitary animals this wouldn't have been the case