Brushing teeth every hour because one's mouth feels dirty could be a sign or characteristic of paranoia or anxiety.
Refusing to eat breakfast for fear of gaining weight could be a sign or characteristic of an eating disorder such as anorexia.
Cutting arms to reduce emotional pain after ending a friendship could be a sign or characteristic of mental illnesses such as bipolar disorder (manic depression), major depression, anxiety disorders (especially OCD or obsessive compulsive disorder), and psychotic disorders such as schizophrenia.