Remember that people who get angry at you are often just upset at something going on in their own life.
Perhaps this is an exaggeration. But the core of the message is that people tend to have stronger feelings about something than they let on. People who regularly have outbursts of anger, depression or flamboyant enthusiasm are generally frowned upon in most cultures. This especially applies to men (for women trying to figure us out). The application of this rule is to not assume everything is fine just because someone isn’t having a nervous breakdown. We all have our individual problems, angst and upsets that are normally contained.