Good question. It's an over-exaggerated fight or flight response. Our nervous system is getting us ready to act by pumping out adrenaline and other chemicals. The problem is this response may be over- zealous or it may occur when we don't want it too. Also if something scared us before, our body remembers that. That is why if you've ever been stuck in an elevator once, you'll get very anxious in any tight space.
Anxiety. Anxiety is often produced by misperceptions of situations or interactions. Try to focus on the facts as much as possible, the free anti anxiety app focuses on this issue in detail with plenty of exercises, articles and the book, transforming anxiety into joy as well as video's to help lessen anxiety. Best.
See below. Anxiety is a reaction to pressure we experience, or can be elicited when anticipating threat/potential threat. We also think a lot about past and present (should've, would've, could've) and this creates anxiety for us, humans. We remember bad or unpleasant experiences and along with thoughts our body will react too ( being in a car accident, will make you anxious to drive in the future, etc.