No one knows why some people get anxiety disorder, but researchers are studying several different things that may be linked to this illness.Problems in childhood
Scientists think that to become a healthy adult a young child needs to feel secure and confident while growing up. That sense of security and confidence depends a lot on the attitude of the child's parents. Some scientists think that if your parents were unable to express warmth towards you when you were young, or if they were overprotective, you may become anxious and unsure of yourself. This may lead to anxiety disorder later in life. Stressful experiences
If you've been through a very stressful or unpleasant experience, you may be more likely to get anxiety disorder. A big stressful event can make it three times more likely that you'll get anxiety disorder. 
The types of stressful events that may cause anxiety disorder include the early death of a parent, very bad marital problems or family relationships, and traumatic experiences such as rape.Chemicals in your brain
Your brain contains many chemicals, called neurotransmitters, which carry messages between brain cells. The amounts of different chemicals in your brain can affect your mood and your thinking. People with anxiety disorder may have too much or too little of some of these chemicals. 
This doesn't necessarily mean that a certain mix of chemicals in your brain causes anxiety disorder. It may be that other things, such as stress, upset the balance of chemicals, and this in turn affects your feelings, making you more anxious.
To learn more, see Your brain's chemical messengers.Hormones
Hormones are chemicals that carry messages around your body in your bloodstream. If you have anxiety disorder, your body may produce more or less of certain hormones. Your autonomic nervous system
Your autonomic nervous system is a network of nerves that control automatic processes in your body, such as your body temperature and how fast your heart beats. If you're anxious, this system may not work properly, giving you symptoms such as sweating and palpitations (when you feel your heart beating faster than normal). Some scientists think that the autonomic nervous system may work less well in people who have anxiety disorder.  Problems in certain parts of your brain
Feelings such as anxiety and fear come from certain areas of your brain. However, we don't know enough about these parts of the brain to be able to say what happens to them in anxiety disorder.