There was something magic about the mid 60’s. We had recovered to some extent from those assassinations. We no longer saw alcohol as the drug of choice. Vietnam was the war that was coming into view. There was a darkness that was balanced by something ever so different. Hair was longer, institutional frameworks were no long taken for granted, the clothes were different with giant colours and bell bottoms along with the return of paisley. All of this had a strong touchpoint – music, it was changing as much as it was new. We had the Beattles, the Moody Blues, the Doors, Jimi Hendrix and Led Zeppelin. This was a new line in the sand. This was the music that had no definitions but the ones we gave them.
To this day that music brings back those moments full of sounds, sights and smells. The context is as important as the melodies. Without the context the music only becomes notes on a page. Nostalgic for sure but when memories surface they are more than mere tunes.
It was a haunting, fearful time were one encountered doubt. But the music was like a sword that cut those fears to the quick and made happiness part of your own humanity.
Tinfoiling 
FWIW, I think it was the adolescence of the Rock and Roll born in the 50s coupled with the emergence of singer-songwriters influenced by both Folk and Blues. It’s easy to attribute it to some mystical zeitgeist but I think it’s the logical out-come of a huge post war population boom and a shift in marketing which targeted the new teenage demographic. I know that this is all very unromantic but few things truly are.