As I mentioned in another comment, I find the lack of personal space to be the biggest anxiety provoking element of flying, which raises my baseline anxiety enough that I can become extremely anxious once you add the relatively small amount anxiety I experience from just flying. Under different circumstances, my anxiety is far lower and I don't cross the threshold into "anxious", for example, when I'm in business or first class, or I otherwise have plenty of room to myself.

I wonder how widespread an increase in flight anxiety is as airlines add seats to planes and increasingly fly planes at max capacity.