Human beings are imperfect creatures, so that every belief that we have is never 100% sure. This is seen by the simple fact that we make mistakes from time to time. At least occasionally, we find ourselves proved wrong on something we thought was true.
In order to go about our daily activities, we unconsciously assume that we are always right. Because if we spend all day second-guessing ourselves, nothing really gets done. If we spend all day questioning whether we really know something for sure, then there is no time left to actually do what we set out to do. Therefore, we simply assume that we are always right. For most people, this is also their best strategy, because they have neither the time nor the capacity to think any deeper about the state of their own knowledge (or lack thereof).
In our daily lives, we only occasionally find ourselves mistaken about something, so it makes sense to go about our activities as though we were always right.
Usually this is fine, but trouble arises when people disagree on something. In their daily lives people are so accustomed to assuming that they are right, so that when disagreements arise, people have a tendency to automatically assume that the other person is wrong.