People become anxious for a few reasons. One, a new experience. Two, new people, you have no idea who they are. Or three, you've just never been out of your comfort zone and you're not used to it. Those are the most common causes of anxiety.
One common cause of anxiety is just simply trying to push it away. Sometimes when people feel uncomfortable and they feel anxiety and symptoms of anxiety, all they want to do is run away from it, which causes anxiety about the anxiety. So the best thing to do is try to embrace the moment and identify the ways you feel anxiety in order to overcome it.
It can be different for everybody. It can be work, being outside, being around people, having too much on your plate, having lots of assignments at school, anything. It could be anything, but anxiety for me is normally caused whenever I have too much that I feel like I have to do. Messy house, plus homework, plus going to work, plus not feeling well. So it can be a lot of things.