The thing I understand least is how I can have one good day and one bad day when all the other factors in my life seem the same. When I am not anxious, I am brimming with confidence. When I feel nervous, I obsess about my upcoming trip out of town. I feel like two completely different people sometimes (in this respect).
To reply to my own question, perhaps the reason is not as important as the re-programming of the brain so we can avoid feeling fearful about challenges that can not otherwise hurt us, such as taking a routine trip.