We don’t. The only way we would know if we were helping them would be if they actually applied the advice and their mix improved.
Audio is a medium that is generally presented without additional explanation. That being the case, that is also generally how it is evaluated by the listener - on its own merit.
The fact is, when, as a mixer I am given a mix to do by a client, my job is to present their work in a way that compliments and enhances their vision. If I explain to the client why I did (or did not) do certain things a certain way, it will make absolutely no difference. The client will still want what they want…
Andrew Scheps talks at length about this many times in interviews he has given. He says something to this effect: “If I’m composing an email explaining my mix to the client in my head while I’m listening back to it, then I haven’t finished the mix yet”. I’m a BIG believer in that philosophy - I identified with it, because that is the conclusion I came to early on when mixing for clients…
…So personally, when I see someone pre-explaining their mix as if it were a purple blob on a canvas in a modern art museum exhibit, it immediately makes me wary. I’m in the position of the client - I’ll hear what I hear; I’ll react to it the way I’ll react. No amount of eloquent prose will make one iota of difference… It all sounds like “uh…sorry teacher… the dog ate my homework”
The question I would ask is “does the mixer’s personal subjective tastes” enhance the song and make it subjectively “better”; improve its “feel”… or whatever? A person’s personal taste in clothing might be to wear crop tops. If that same person has a giant beer gut, is anyone in their right mind going to encourage them to pursue that mode of clothing?
The fact is, there is a wide variance of style among the top mixers of today. But anyone who is a student of music can quickly tell whether the person who mixed the song knew what they were doing or not. I think even someone who is just starting out in audio (& I’d venture to say even “the average listener”) has a sense of that too - what sounds “pro” and what doesn’t…
It’s much like a “dress code”. If the dress code for an establishment states “smart casual”, there is still a lot of room for variance according to personal taste. But if someone walks in with no shirt and flip flops, they’ll stand out like the proverbial and be easy to spot.
It may just be one of many small decisions that add up to something that “feels” better as a whole. Whenever I critique, the very first thing I listen for is “does it feel good”; “Does it feel right”. If it does, I’ll say so. I might give it another forensic listen to see if there are any technical things that could be improved, but generally speaking, if it feels good, those things will be low priority.
On the other hand, if it doesn’t “feel good”, I start asking myself “why”. More often than not, the reason is a technical one - The vocal is too dynamic; the mix is muddy; the kick drum isn’t driving the groove as it should; the low end of the guitar is covering up the top end of the bass… etc etc.
Sometimes, it is a matter of taste. For example, I personally hate mixers whacking chorusing on guitars just for the hell of it. I generally try to make it clear that I’m making a comment that reflects my personal taste. Each to his own.
Don’t forget, critique is every bit as helpful to the person doing the critique as it is to those being critiqued, IMO. This is the way you practice your critical listening skills, which I would venture to say is THE most important thing to develop in this field. When you listen carefully and critically, you can’t help but develop a viewpoint; a perspective as to what sounds “good”. As @ingolee so succinctly put it:
Amen. In mixing and producing, taste is everything - it is why one person gets a project and another doesn’t.
Develop your opinions, develop your taste.