I would guess that the part that caused such a strong reaction was this:
because I find it almost excruciatingly awkward to sit right next to somebody for hours without any communication except for quick glances. Why the hell do people do that?
You're not just introducing yourself: you are putting pressure on the other person to be social, both with the notion that you would find sitting in silence "excruciatingly" uncomfortable, and with the implication that a lack of communication is unusual and unacceptable.
Usually if somebody would introduce themselves and try to start a conversation, one could try to disengage, either with a polite "sorry, don't feel like talking" or with (more or less) subtle hints like giving short one-word responses, but that already feels somewhat impolite and is hard for many people. Your opening makes it even harder to try to avoid the conversation.
Hmm... good point. What I typed isn't exactly what I usually say, but I do tend to project my personal opinion that sitting quietly side by side is awkward and alien (to me) behavior. I can see how conveying that impression makes it difficult to disengage. And while I do find the silence pretty damn awkward, other people have no obligation to cater to my hang-ups, and its kind of unfair to (unconsciously) manipulate them into that position. So on consideration, I'm retracting my initial post and reconsidering how I approach these conversations.
r/Fitness does a weekly "Moronic Monday", a judgment-free thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. I thought this seemed like a useful thing to have here - after all, the concepts discussed on LessWrong are probably at least a little harder to grasp than those of weightlifting. Plus, I have a few stupid questions of my own, so it doesn't seem unreasonable that other people might as well.