The speaker compares touching an unidentified person's "goodness" to a person nervously handling an unopened letter because it is the first letter he has ever gotten from anyone. The nervous reaction is stimulated by a combination of fear about what the letter might mean and embarrassment because the only he will learn what the letter says is to ask someone to read it to him.
The simile continues as the man holding the letter imagines various scenarios that may be result from learning its contents: maybe notification of an inheritance or possibly word that his parents have died. Or, maybe, the letter might just bear news from a "dark" girl that she has had a change of heart and decided she does love him.
Fear of revealing he can't read the contents and pride at finally having a letter of his very own for the very first time leads to the decision to leave the contents unknown. Left unread, the letter leaves him rich, orphaned and loved. What, he wonders, would the unidentified person whose goodness he has touched call the emotions he feels for unseen words capable of leaving him in such a state.
