Never lived in LA (been there for a few hours once), so I don't know. In Bay Area I lived in Berkeley for about 4 years and occasionally visited San Francisco and the the South Bay for entertainment etc, I would say the cultural atmosphere is similar. As I said I was talked to by strangers on the street (and especially in a market e.g. Trader Joe's, Safeway, The Dollar Tree etc...) on multiple occasions.
Currently living in Boston, MA. It's definitely not as usual as the Bay Area, people are significantly "colder" (for the lack of a better word) but I wouldn't say talking, or smiling to a stranger would be as weird as in Europe. Maybe LA is different than both the Bay Area and Boston area (although Boston being in itself significantly different than the Bay Area in the first place)
Currently living in Boston, MA. It's definitely not as usual as the Bay Area, people are significantly "colder" (for the lack of a better word) but I wouldn't say talking, or smiling to a stranger would be as weird as in Europe. Maybe LA is different than both the Bay Area and Boston area (although Boston being in itself significantly different than the Bay Area in the first place)