Pain

What is it about the human experience that makes us cause another human pain. I'm not talking about anything like violence or war or even teasing someone with the intent to hurt. I'm talking about the normal things people do in their lives that end up hurting someone. The things that are unintentional. Most people don't go around trying to bring pain to someone else, but it happens all of the time nonetheless. Why is that? How can so much anguish and grief come inadvertently?