There are many causes of anxiety. Usually the causes for an individual developing an anxiety disorder can be from any one of the following:

Environmental factors.
A stressful job that you simply cannot tolerate anymore is a major source of anxiety, as is being unemployed and trying to desperately seek employment. A toxic personal relationship can also be a massive source of stress and anxiety. School or college can be a massive cause of anxiety, particularly during exam periods or if a student is being bullied.

A traumatic event can trigger post traumatic stress disorder, she as being assaulted, being in combat or even witnessing a stressful event. Even a shortage of oxygen in high altitude areas can bring about anxiety.

Genetics
Genetics can play a big factor in one developing an anxiety disorder. If a parent has anxiety then there is a high probability that offspring will

develop an anxiety disorder.

Research has shown that children who are emotionally neglected by their mother whilst growing up, will have a lower tolerance to stress, hence will likely develop and anxiety disorder and pass this onto any children that they may have.

Medication
Certain prescription medication can have certain side effects that cause anxiety, such as antidepressants. Even illegal narcotics can such as cannabis can cause long lasting brain chemical changes resulting in depression and anxiety.

Brain Chemistry
A lack of the neurotransmitter serotonin can cause depression, certain drugs and substances such as Caffeine and Nutrasweet can lower the levels of serotonin in the brain resulting in anxiety.

Withdrawal from illicit substances
Withdrawing from illegal substances such as heroin and cannabis can result in depression and anxiety, due to the reduction in the levels of the neurotransmitter serotonin.

The most common cause of anxiety comes from the stresses of day to day living and any of the factors mentioned above. It is usually a subconscious response to outside factors, it is also possible from an individual dwelling on negative thoughts, convincing themselves that something is going to happen.

Anxiety can occur from a combination of the above factors, for example somebody may respond to stressful situations by drinking excessively.

A mental health professional is usually best placed to make a diagnosis and identify the possible causes of an anxiety disorder. Symptoms must interfere with everyday living causing an absence from school or work. If no specific cause is found then a diagnosis of generalised anxiety disorder will be made.