Personality disorder
There are 10 types of personality disorders, including schizoid and narcissistic, with around 4% of the UK being affected by at least one of them.
A person suffering from a personality disorder will experience feelings and emotions which are far different to people around them, such as anxiety and paranoia.
For example, people who have developed a paranoid personality disorder will experience severe feelings of distrust towards others; even close friends and family. They may also assume people are constantly looking at or judging them, and will constantly be hunting for validation of their fears.