As well as environmental factors, I think it's closely related to the placebo effect too, which strongly believe is massively underestimated by the audiophile community and massively exploited by the hifi industry.
The human mind and senses are extremely suggestible, especially our hearing. Our enjoyment and opinions are effected by things such as mood, tiredness, concentration levels, expectations, social factors, personal preferences, etc etc.
I find music sounds better in the evening, but i put it down to being more relaxed and comfortable. I once demoed some gear one Saturday morning with a friend. The next day we revisited the store and asked for another demo. We both agreed that this gear sounded much much better than what we heard the previous day, only to find out that it was the exact same gear and CD with heard the day before. The difference was astounding, yet nothing had changed. That was quite a few years back and since then i have read up a lot of the subject and have spoken a great deal to a relative who works as a sound engineer for the BBC and also worked at Abby Road for a short time, they spend thousands eliminating bias. He, and other engineers, are also extremely cynical about the high end audio industry.
*other opinions are available.