-Humans, for all sensations, will eventually cease to consciously perceive things as actively as when they first heard them. The brain decides that the environment is normal and does not bother making you think as hard about it. This is why some people continually turn up the volume to ear-splitting levels. There is a temptation to turn the volume up because that will reengage the brain to think 'the environment is different, this could be important.' It isn't a matter of hearing details you can't hear at lower volumes.
-Additionally, when one lowers the volume, a similar reaction is produced in the brain (see point above). Normally this is immediately perceived as bad, but this behavioural, not a necessary truth. If one listens at a low volume, one can force his or her brain to be active which can increase the enjoyment of music, as you are consciously trying to hear the details instead of letting them be slapped into your face (and as previously mentioned, you eventually get used to this slapping anyway).
It's interesting about volume. The biggest trap I find is if you go from listening to a loud track to a quiet one, turn the volume up and then don't turn it down again for the next track. Going from electronic to acoustic music always sounds bad.
Weirdly I find I need to listen to music louder on work days than weekends and holidays, even if in exactly the same environment. That must be psychological, maybe something to do with the brain being less engaged but I'm just speculating. Anyone else get that?