The first critical issue here is an ethical one. Shyness is a virtue and duty. Why? To disturb others, to say hi to a stranger, is seen as a bad thing.
“I’m sorry to bother you but…â€
You are not a bother!
Often the biggest issue we do is we think we are bothering or troubling others. No! Your injecting fun excitement into their life! 
Second, just practice. Whenever you walk by a stranger, just wave your hand at them!
I think the critical issue here is that we are somehow shy and afraid to acknowledge the existence of others?
Experiment
I am shocked, we could see somebody from a very very far distance. I do an experiment, for the people I already know, I always throw up the peace sign at them, and they almost always universally way back at me, even if they are half a mile away! 
I think to experiment and look closely at somebody, from a distance is a good idea, because often the biggest issue that we do is that we don’t acknowledge their existence.
But what do you do if you acknowledge the existence of somebody, let us say your cowardly neighbor, and they never acknowledge your existence, and pretend to hide from you? I say keep saying hi to them, 1000×1000 — the Jesus way!
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