Some good comments made so far.

Another thing to understand is that there are lots of different settings that can be applied to the mastering process. If the mastering engineer is using a particular mastering software it can have a bewildering array of choices, and only a few might be relevant to your music. If the mastering engineer is doing it 'manually' then a good understanding of the needs of the musician is essential - and the musician should be consulted during the process.

When Doug Bailey and I mastered the last Old Swan Band album at Wildgoose, we re-mastered four times (if I remember correctly) before getting the required result. Even then, the tracks didn't sound as fresh and vital as the original raw mixes but they would be more consistent on radio and on a variety of playback systems so it was worth doing rather than not.

I've had stuff mastered in my absence in the past, especially in the old days when I recorded for Decca and the producer 'knew best'. It can be demoralising.