A person's condition is diagnosed based on the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV). "The DSM lists specific and concrete criteria for diagnosing nearly 400 disorders in children, adolescents, and adults" (Ellen Pastorino & Susann Doyle-Portillo).

What causes psychological disorders?

This has been the question facing medical practitioners, theologians and psychologists for years.  Since then, they have grown to understand these disorders through a cohesion of multiple theories.  Diagnoses have been known to fall under more than one theory.

Theories, according to "What is Psychology? - Second Addition":

• Biological: Abnormal behavior is due to biological, genetic, and physical processes such as an imbalance in hormones neurotransmitters.
• Sociocultural: Abnormal Behavior is a result of social and cultural influences.
• Psychological:
  • Psychoanalytic Perspective: Abnormal behavior is due to unresolved unconscious conflicts.
  • Social Learning Perspective: Abnormal behavior is due to learning processes like classical and operant conditioning.  It happens in response to environment.  
  • Cognitive Perspective: Abnormal behavior is due to thoughts, expectations, assumptions, and other mental processes.
  • Humanistic Perspective: Abnormal behavior results from distorted perceptions of the self and reality. 
    

Sources:

Pastorino, Ellen, and Susann Doyle-Portillo. What is
Psychology?. 2nd ed. Belmont, CA: Cengage Learning, Inc,
2009. Print.