Explicitness - A Choice Word

In reading Johnston's book, Choice Words, I find myself highlighting many sentences and talking to myself - "Oh, I've done that" or "Oooohhh, I shouldn't have said that to my kids" or "Wow, I never thought about how that (what came out of my mouth) could possibly be received."  In Chapter One, under the "Explicitness" heading, there is a particular sentence that caused me to have much retrospect, "...the better you know something, the more risk there is of behaving egocentrically in relation to your knowledge.  Thus, the greater the gap between teacher and learner, the harder teaching becomes."  Even though the implication is on academic teaching, I believe this applies to one's life in any area that you become the teacher.  I think about my own life - I have hired and taught young marketing people, I taught my own children and I continue to teach young equestrians on riding. In contrast, I have been the learner at times as well - as a child, as a young professional and still today working with professional trainers in the horse industry.  As a teacher, I know there are times that I wonder "Why don't they know this?  Isn't this everyday, common knowledge?"  And as a learner, the questions have been asked in reverse, "Don't they know I don't know this yet?  What do they think this is - common knowledge?"  I have had equine instructors that were at such a high-level in the game that they could no longer teach someone that was not at their level and the similar has been true with academic professors - they had been teaching the subject so long and invested so many hours in it that they could no longer relate it to the average student.  And I find myself doing this with the things that I have invested YEARS AND YEARS of time in.  So, how do you as a teacher - whether it be about life, academic or even parenting - decide what to be explicit about and what to let the learner learn on their own?  I'm not sure, but I know that Johnston's Choice Words at least gives me pause for consideration of my own future actions.