What causes one person's anxiety may not create the same response in someone else. Anxiety can be brought on by different situations or experiences.
Anxiety can be an understandable response to a stressful event in your life, for example:
Sometimes it can be difficult to know what is making you anxious. This can be upsetting or stressful. If you learn to recognise what is making you anxious, it can help you deal with the uncertainty.
Anxiety can also be part of stress. (HSE website)