Life isn’t fair.
That being said … Most of the time I hear someone say the phrase “life isn’t fair” it is merely used to justify assholery.
Teachers, parents and other central planners decide certain aspects of how people, usually children, are treated. People like to think these people are objective, considerate, and have a sense of justice in how they treat people. When a person believes that this person of authority violated their implied objectivity, consideration, or sense of justice … it will often come in the form of telling them that they aren’t being fair.
In response the “authority” tells them, “Life isn’t fair”. This is akin to admitting that you have no sense of objectivity, consideration and sense of justice. In this moment, the child feels like he has to heed what you say, not out of a sense of principle, or ideology … But rather just due to the fact that you have power and you really just told him that you exercise that power arbitrarily based off of your whim.
Sure, people can say something isn’t fair when it actually is fine given the circumstances. This makes for a great opportunity to learn, grow and communicate with people and to show them the boundaries of your beliefs and power. Saying “life isn’t fair” in this circumstance is just being an asshole and you will lose the respect of the people you are leading.