There are a number of reasons why some people suffer from excessive anxiety, but in many cases, anxiety begins at a young age and develops, through repetition, into a habit. Often unhelpful patterns of thinking and dealing with emotions are learned from our parents and other adults.
Essentially, anxiety is created and reinforced when we resist experiencing our emotions. For example, if we are afraid of experiencing anger, then whenever any hint of anger arises in our minds we will try and push it away, and this produces a strong sense of anxiety. Each time this happens, we creates a pattern or habit in our mind, so that whenever the feeling of anger arises, we are likely to resist it and will experience anxiety instead.