In his wonderful book Information Anxiety 2, Richard Saul Wurman gives one reason why: “Once you know how to do something, it’s almost impossible to put yourself in the place of someone who doesn’t know.”
Not convinced? Then try telling a 7-year-old how to tie her shoes.
Another name for this problem is the curse of knowledge. Wurman suggests a solution — link to prior knowledge:
When you are presenting information to your audiences, remember that although you’re exceptionally familiar with the topic, they may not have any idea what you are talking about. In every presentation, begin with something familiar. Give your audience at least one fact they already know and tie that into the material you are presenting. Give them something slightly familiar so they have a starting point, an initial connection to the new world that you’re bringing to them.
I’ll add this suggestion: If you’re writing a book, seize every opportunity to explain what it’s about. For best results, gravitate toward people who know nothing about your topic.