Behavior does not always have to be an action. After a tragedy, people can become anxious and fearful. This behavior spreads as well—leaving many people fearful and anxious. Have you ever felt like not being around a friend of yours because he or she is just too sad or too negative? You do not want to be in the presence of this person because you know that their negativity can "rain on your parade" and make you negative or sad too. Social psychologists refer to this as mood linkage. So, if you are happy hanging around Happy Harold, there is a reason you want to always be with Harold!
Mood Linkage: This phenomenon states that our mood is linked to the people in our company.