I don't know if you've ever mixed music, but it takes many hours to achieve a good balanced mix. In those hours the mixer spends most of the time replaying the mix paying attention to every little part and how it fits together in the mix. Adding effects, splicing here and there, playing with volume, panning etc.. Anyone who has spent time mixing, will tell you to put it aside and listen to it the next day (or start over). You focus so much on specific parts of the mix, that you don't hear the music as a whole. Sometimes we just have to sit back and listen to the music.

A good mix is the product of a process. As the process continues we forget the mix and go back to it. A mix is never really finished. We listen to it with new ears again and agian, even though we've heard it a thousand times. A good mix should be balanced. Of course good and bad are relative. Preference and musical direction play a part.

Many times the instruments are recorded or played terribly, so you have to start all over.

Sometimes we think it sounds good because it has so many effects to make it loud, but really we are fooling our ears and it is in fact dynamically and harmonically difficient. It sounds good for the first minute, but after that we want to change the channel.