My friends often ask me now what happened to that person they used to know. Now, for whatever reason, I can get very anxious in crowds. The last time I was in New York, I insisted on riding in taxis, much to my friends' chagrin. I couldn't stand the thought of going into the underground filled with swarming people. You could say that I felt anxious about taking the subway, particularly with holiday crowds and the possibility of mob mentality taking over.
The change happened very quickly, about 3-5 years ago. At around that time I had gotten the H1N1 virus and was sick for many months with secondary infections. My immune system got so worn down that I could hardly be around anyone without picking up whatever germs they had and getting sick again. I think it was then that I started to see everyone as a potential threat. And for the first time I realized how vulnerable I was. My health turned out to be something that I couldn't beat or cheat. I wasn't myself at all. It was disturbing. Ever since then, I can get anxious.
More than the flu, though, I believe that I just happened to have gone into the elbow of my exponential shaped learning curve. For the past decade I had been forcing awareness, probabilities and the assessment of risk into my conscious mind so much, I think eventually I just got so good at detecting and accounting for risk that I am just overly aware of things that most people choose to ignore.
I will give an example of what I mean. When I drive long distances, to fight my ADD tendencies, I make myself hyper-aware. If I closed my eyes, I would be able to recreate the exact scene in front of me, where every automobile is, a projection/guess of where they'll be a second from now or 5 seconds from now at their current speed, taking into account people entering or leaving the roadway, constantly aware of even the slightest possible outcome and constantly refreshing that information with Bayesian updating. It makes me a safer driver, but it also puts me on edge. I get a little Funes the Memorious about it.
Now that I've forced myself to be so hyper-aware of risk and probabilities of future occurrences, I feel like I cannot un-see them. If there are a lot of things going on, it almost overloads my system. Although I can predict rational human behavior, humans are not consistent and when they deviate from normal behavior it's usually when the stakes are highest, making the potential collateral damage to me high. And now I have more to lose -- more money, a more stable life, a career, a relatively stable set of close associates. All these figures get crunched in my head and make me aware of a million different risks, which added together are not negligible. And that awareness gives me the symptoms of what is probably best described "anxiety," even though I used to be completely oblivious to all of it (or didn't care).
I may still seem reckless, particularly in circumstances in which people are irrationally afraid and I am relatively unfazed. I still like excitement in my life; I tend to seek out new and potentially dangerous experiences. But as I have aged, I have admittedly retreated into more a life of the mind in which my excitement and thrills come more from mind games or intellectual pursuits where the reward/risk ratio is high.