We Know Little About Others
We only know a small part of most people's lives. In many cases, that's fine. We should just be mindful of this and not try to assume that we know everything about someone based on what we see or hear.
This is one reason why we shouldn't judge others. We don't know about their struggles, motivations, or circumstances. We know too little to make a judgment.
From another point of view, this fact is a call to deepen our friendships; to leave the shores of the banal and superficial and go—as Aristotle would say—beyond utility or pleasure, and appreciate the other person for who they are. This usually takes time and doesn't happen automatically. In the process, we will learn to appreciate and celebrate our differences, not just our similarities. We will become more empathetic, understanding, and generous. We will become better friends and better people.