Where did early/mid 2010s optimistic millenials come from? They had no reason for it, america was still at war with iraq/afghanistan, the 2008 finacial crisis crippled the global economy; so where did they fucking come from?
Maybe because they were raised in better times and saw older relatives who had "made it". Plus they were still relatively young as they had barely turned 30 or were still in their 20s
I was fairly young (I'm a zillenial) but I was already online and the vibes were "yeah things are bad but Obama and the culture he represents are leading us out of it". Keep in mind that Bush was a pretty widely loathed president by the end of his term and he was the one who started all that shit. American conservatism and all its adjacent ideas seemed condemned to the dustbin of history. Young people would ensure that each year would be better than the last.
This is why the first Trump term was traumatic for libs even though it wasn't nearly as insane as his second one. It shattered that simple-minded idealistic worldview.