No one knows why they are. They only know what they are. And that is an energy pattern that exists in the body for a time. People who are mean feel that they need attention. They want to feel important or powerful. As adults we all have to support our own ego. No person or group of people will always support or cheer anyone on because they have their own needs. So the mean person attempts to force people to pay attention to them. It is due to a lack of contentment, inner peace and self love or sometimes it is a coping mechanism because they have been mistreated by others in the past. They treat others how they have been treated.