I had never been good at judging others although I had been a student of Psychology for many years. I have come to realize that no bookish knowledge can be beneficial for you until you get to have some practical experiences related to it. Whatsoever we read in books does not seem to go into our heads until we get a first hand experience of it.
It is expected of a psychology student that he/she might be having a good understanding of the human behavior, but when real life situations occur whereby one needs to have such an understanding of the persons around we require some period of time to achieve it.
You actually need to get interacted with the person and that too for a good period of time to get a glimpse into his/her nature and behavior. It is possible only over a time-period that one really gets to know people.
Otherwise, only a part of the personality is revealed to us. When we actually live with someone then we know him better. Also, long-lasting friendships and marriages are good examples of how people get to know each other well and then find happiness together. They know each other's weaknesses well so they get along taking each other's faults and good points.
Some aspects of people's personalities can be very obvious and for that we do not need much time to understand them. Some behaviors and expressions are so apparent that you can easily tell if a person is of what type. But as we do not always take so much time while interacting with each and every person that we meet in daily life we tend to take them at face value and they do the same with us.
Hence, the routine courtesies and manners play a vital role in making us acceptable and welcome in other's zone and vice versa.
General behaviors and expressions are sometimes skin deep and we have to take others at face value and the true nature of the person lies inside him, unrevealed, hidden.