No one knows for sure. There is evidence that some people are born more prone to anxiety than others, and evidence that societal factors play a large role as well. Parents with a tendency to be overly anxious can pass that tendency on to their children. Unsupportive or abusive family lives can cause the development of anxiety disorders as well. Still others develop anxiety disorders because they are exposed to a traumatic event like an accident, natural disaster, or severe abuse that makes them question their basic safety. As a general rule, studies indicate that women are twice as likely to develop anxiety disorders as men.
Finally, there are some physical conditions that make people act like they are having panic attacks or anxiety. Among these are hypoglycemia, hypothyroidism, hyperthyroidism, mitral valve prolapse, inner ear problems, congestive heart failure, deficiencies of certain minerals and vitamins, withdrawal (like nicotine withdrawal) and PMS.