Let’s begin with a simple graphic explanation of anxiety. First, consider the concept of fear, which must be distinguished from anxiety. If you were sitting in a room and suddenly a large rattlesnake crawled through the door, you would have good reason to be afraid. That’s fear because it refers to an actual threat. Fear, in some cases, can be healthy because it often keeps us alive.

But if you were always worried that a rattlesnake might crawl into the room, even if no rattlesnakes were anywhere in sight, that’s anxiety. Anxiety is most often not helpful because the threat is imaginary, and a lot of time and energy can be wasted worrying about things that might—but not necessarily will—happen.

The psychological basis for anxiety can usually be located in childhood experiences that lack clear explanations and guidance from parents who tend to be disinterested, critical, or abusive. Hence the children grow to dread circumstances that have unknown or unpredictable aspects.

Some individuals live with a constant, general sense of worry and anxiety, as in Generalized Anxiety Disorder. Typical symptoms are tension, restlessness, fatigue, irritability, sleep disturbance, and difficulty concentrating.

Other persons feel a more focused anxiety, as in a Panic Attack, where their heart beats faster and faster, and there is a sudden onset of apprehension, terror, or impending doom, to such an extent that they might feel they are going crazy—or having a heart attack. In fact, it’s quite common for patients to appear in hospital emergency rooms complaining of having a heart attack when they are really experiencing a panic attack.

Source: http://www.guidetopsychology.com/reasons.htm#Anxiety