Why does the sound sound better at higher volume (loudness) ?
Well, the ear perceives all frequencies equally strongly. It could be shown by Flethcer and Munson, two researchers already in 1933, with their measurements. 

How to understand Fletcher-Munson graph?
The Fletcher-Munson graph says that at stronger sound pressures, the ear's ability to perceive is leveled off in the way that we perceive all frequencies as equally strong i.e small red areas in the diagram i.e. that with strong sound the experience becomes richer. The treble is roughly the same regardless of loudness.

To read the graph, remember you should look only at one curve at the time!
For example, you want to know the ear frequency perception at loudness*) at 50dB at 100Hz and 10kHz. Find the curve that hit 1 kHz at 50 db (middle yellow arrow) and then follow the curve to the left 100 Hz gives 18 dB (red area) i.e. the bass requires stronger sound pressure +18db compared to a freqency of 1kHz. To right 10 kHz treble +10dB compared to a freqency of 1kHz (red area).

IMPORTANT! Each curve shows the threshold for how weak sound an ear can perceive at a loudness*). So frequencies below the curve (red areas) are sounds (frequency) that we cannot perceive at a given loudness*).
*) The reference loudness of the curves is defined at 1 kHz (yellow arrows).

At 80 dB loudness curve we can see all frequencies as equally strongears (green area). The treble is roughly the same regardless of loudness.

What level shall I mixing & master to get it right?

Difficult to give a short answer, but a one rule is to start level around 80 dB, the green area in the graph above. At that level 80 dB SPL the ear respond it’s not quiet, but not painfully loud either. It’s the best level for hearing the frequency spectrum as balanced as possible.

For those who want to know more Click HERE 
Take care of your ears - Always listening at a low volume!

I hope you may find this page useful,
Very Best, Jan Karlqvist, creator of CubaseINDEX.com