If I had to pick from the four reasons you're saying, I would say (c) and probably (d). But that doesn't mean that they're really the causes; it just means that out of the four things you said, (c) and (d) seem more likely than (a) and (b).

Even if (c) and (d) are the reasons, though, that doesn't really shed any light on it. I suppose everyone gets bored, and I suppose that everyone's brains would produce endorphins if they did this. Yet not only do most people not practice SI, I am apparently atypical of even those who do practice SI.

Thanks for the advice. I don't find everything here unhelpful, I just find it unhelpful when it's all blown out of proportion. I'm already inclined to overreact, and I don't need anyone's help with that.