Is it a cultural thing in US to answer back hello?
Yeah, people greet strangers in the streets here (as well as on the train and on the bus, though it feels like this is less true over time) and it's a major faux pas not to reply back. Most places you're marked as merely rude and they go their way and forget about you. In a few regions, you're regarded as suspicious, and in some really backwoods places you're marked as hostile and possibly trying to pick a fight.
I've only been to six countries where I stayed for longer than one month, so sample size is very small, but it *feels* like an American or at least North American thing. This is not really a thing in the biggest cities (but it's still a thing in the medium-big and medium cities and below)
Yeah, people greet strangers in the streets here (as well as on the train and on the bus, though it feels like this is less true over time) and it's a major faux pas not to reply back.
However, in no case should you ever answer "how you doin'?" honestly. You just say you're doing OK, no matter what. It isn't really a question to be answered, it's just a social noise.
