This is only relevant to about half of what you're saying, but I'll say it anyway.
The reason why I'M afraid of greeting or talking to strangers is because, well, I know little to nothing about them.
Now, for those of you who are still polite and friendly to them, I have to thank you; there are people like that I see on occasion and around them I feel at more ease because of their openness.
But the reason I'm afraid of doing so is because of what I said earlier, and let me evaluate; since I know nothing, that means almost anything is possible (to a human degree); I'm paranoid of who they might actually be, and this is pretty much just an issue with me, because I have a tendency to consider all possibilities, which means that I assume that the person standing in front of me could be anything from a criminal to a saint. I know it's absolutely ridiculous, but to this day I'm still like that, and it's the reason why I'm very cautious in large crowds or near few people; I'm worried that someone might have some sort of motive by saying hi to me, or that someone in a crowd might try to pickpocket me (reason why I frequently keep hands in pocket).
I'm working on making myself less paranoid, but that's what it's like for me right now.
Now, assuming that, say, other people are worried about who the strangers might be, and a lot of them think like that, then the aura amplifies; because they are as cautious as you are, you feel you have even less of a good reason to greet them.
That's just an assumption on my part, anyway.