I wonder if part of it is to do with the march of technology, and the developements in Sound reinforcement and quality. Back in the '50s and before, in fact right up to the mid '60s an amplifier of 30-50 Watts was considered really big, and until the late 60's 100 Watts was almost unheard of !!,Nowadays with big P.A systems operating in megawatts, it is perfectly possible to produce good quality sound at a much higher volume than previously.
Younger people have grown up with the expectation that live music will be LOUD !! whereas the older generation still remember the days when such volume was technically impossible to achieve, and consequently tend to expect music to still be at the volume that they experienced in their youth. In other words your expectations of what is normal and comfortable in terms of volume are conditioned by your earliest experiences of listening to music.