I think this 'good vibes only' aspect of US society is a side-effect of the country being only a few hundred years old and never having dealt with any real existential catastrophe. 9/11 is probably the closest thing to one the US has undergone, and I don't think the country has handled it that well.
Edit: I considered the Civil War, but internal strife is simply not the same as an external/natural disaster that overtakes the whole country.
Im also curious what existential threat in say the UK shapes peoples world views today? And more specifically How it crushes optimism?
I didn't say anything about 'crushing optimism.' Good vibes only is a contemporary American idiom that essentially says 'don't bother me with any negative opinions.' This can be a mild desire to socialize with chilled out people, or breezy way of indicating you don't give a shit about other people.
Americans in particular are likely to be put off by something that sounds like it's questioning their life decisions.