I was just about to make a thread on this topic.
Often times I just don't ever think to talk. It's almost as if I forget I have the ability to. If I need to get from point A to point B and someone is standing in my way, I'll wait patiently for them to move. I rarely ever think to say "excuse me." Same with group conversations. Unless something is directed specifically at me, I usually won't ever say anything because I don't think to. I'll listen and pay attention to what's going on, but unless something absolutely must be said, the thought never crosses my mind.
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Remember, all atrocities begin in a sensible place.