The person who underlined a sentence may have been answering a question the next reader cannot see.

Marks in a book are half a conversation. The underlining shows the answer, but the question that made that sentence feel urgent belonged to a life you know nothing about, a worry that person carried into the chapter. You see the emphasis and not the need, which is why other people’s highlights so often look arbitrary.

Hold that in mind the next time someone’s reaction seems out of proportion. What looks like an odd emphasis usually means the thing landed on a question you cannot see. Rather than judging the response, you can ask what it touched, and most people, if asked gently, will tell you.