Loneliness

Loneliness is a sad emotion that a person can feel if they are isolated or feel a sense of lack of friends and family members. Loneliness can make people feel anxious.
People can feel lonely even if they are surrounded by a lot of other people. Loneliness can occur even to people in marriages, relationships, families and successful jobs. One cause of loneliness is a lack of friends during childhood and teenage years. Loneliness can also be caused by chronic depression. Loneliness can also be caused when a relationship or marriage ends. 
Moreover, in the scenarios where you are the only who understands the whole picture, the person might learn few techniques and read self-help guides that can help in coping up with loneliness. 
Loneliness is a theme in literature, going back to the Epic of Gilgamesh.