I often tend to underreact to my environment, too. A prime example of this was when I was brushing my teeth—rather vigorously—when the tooth brush slipped from my mouth and jabbed me right in the eye. Toothpaste burns! I said ow and was worried about whether the chemicals of the toothpaste might ruin my contact lens. I walked out of the bathroom back into my dorm room so that I could take my contact lens out; then I rinsed my eye.
In conversations, I usually have to force myself to smile so that I don't look disinterested or unfriendly because I have a flat affect most of the time. For example, after my statistics class, I was talking to that (probable) obsessive-compulsive girl who borrowed my notes. I had to remember to make a smile when I said good bye to her so that I wouldn't appear too uninterested in her. It's hard; she doesn't generate too much enthusiasm because she's very businesslike, and I don't even have a clue whether she's interested in me or just being friendly in some way, and I still don't know her well enough to have developed much attraction to her.
A third example is riding my bike to get around campus. Someone once noticed that I very nearly ran into a car and just went on. They found it odd that I wasn't shaken from this. I was for a few seconds, but I still had a class to get to.
Most of the time, I feel as though I'm in a state of waking sleepfulness. I'm awake and aware of my surroundings but not feeling completely vivid, I suppose is how one might describe it. Everything seems dull and distant. For a few moments after I exercise, for example, though, things can seem sharper, more brilliant, and more immediate as, perhaps, more blood rushes to my head.