> (Boston) .... Why exactly would it be _dangerous_ to wave at someone in a friendly way?

It's not dangerous at all to do this. It's just considered odd & borderline impolite to do that. It's hard to explain to an outsider, but you see it brought up a ton on places like r/boston. The stereotype is that people in the northeast are "kind but not nice". By and large we don't engage in frivolity like greeting random people when walking around.

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jen20 5 days ago | parent [-]

This is also true of the tube in London - if you talk to a stranger on the morning commute you may as well have peed in their backpack - the response is about the same.

	▲	matwood 5 days ago \| parent [-]
		And it can make sense. A lot of people are packed in small space and commuting. Not exactly an uplifting part of the day. I can understand it's considered polite to try to keep quiet. What's funny is where I live now in Italy (outside of the big tourists areas) is the exact opposite. Any line or idle time you have with random people will become a conversation. It's almost weird to not have a conversation.