Listening Too Loud
When listening to music that is to be mastered, you want to be listening at a comfortable volume. Don't listen at a loud volume like you are at a club or something because for mastering, that can hide what's actually wrong with a song simply by it being too loud.
A good reason to not listen too loud is that your ears could end up with early ringing or a more hindering problem where you lose a threshold for hearing certain frequencies. Tiring out the ears early in the mastering process can lead to less than optimal ability to hear what's going on in the music. Oh sure you will hear it, but not in the way it needs to heard for the job. Furthermore, if you have to turn the volume all the way near 10 to get loudness, that is a sure sign for the need for mastering.
The mastering engineer is going to make the song loud in the process without turning up the main volume. The goal is about making the right adjustments to achieve a perception of loudness at a lower volume setting. So there is no need to turn up the volume anyway for that matter.
Bottom line is that listening too loud can distort the listening process to begin with. A proper mastering session is always done at moderate to low volumes with gradual gain increases to reach a final loudness result.