Gah, I often have that problem. I usually do respond at this stage in my life. When I was a kid, I used to ignore people's greetings even when they were obviously talking to me, because I just didn't feel like interacting. This drove my Mom crazy, and she taught me the importance of returning greetings. These days, I respond to greetings more often than not, though I will sometimes look around to see if there's someone else in the general vicinity the greeter is likely to be speaking to. It's especially difficult, because I have sometimes actually been greeted by people whose faces I don't recognize. Thus, I can't always tell whether or not I've met the person who's given the greeting. If I haven't had the chance to establish a degree of familiarity with a person, I most likely won't reliably recognize said person's face.
I recall a time within the first couple of weeks of my freshman year of college. I was walking somewhere, and a girl said "hi." I didn't see anyone else there, and I didn't know whether or not I'd met this girl before. I returned the greeting. The girl stopped in her tracks for a split second, remarked, "Odd," then turned to a person directly behind me, and launched into a conversation with that person.
If only that were the most awkward moment from that time in my life.
I've had plenty of similar moments. That one stands out, primarily because that was a time in my life when I was trying really hard to start over and reinvent myself as a "normal" girl. It became really clear I was failing miserably when the head of Student Life callled to inform me that several of my suitemates had come to him about me; that there were things I'd been doing which caused people to be "concerned" about me.
But I digress. My point is, I can definitely relate to this.
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"And I find it kind of funny, I find it kind of sad./ The dreams in which I'm dying are the best I've ever had."