Symbolism, Imagery, Allegory
The characters in Two Gentlemen of Verona pass more notes than a class full of sixth graders. They also pass more letters than in any other Shakespeare comedy. So, what's the deal with all the letters in Two Gentlemen? Well, the first thing to notice is that writing and passing love letters is a major component in the process of wooing (a.k.a. flirting with the intent to marry). In the play, if you want a relationship with someone, you whip up a little note that says something like "I like you. Do you like me? Mark one box – yes or no." Or if you're Silvia, you trick Valentine into writing a love letter to a "friend" so you don't seem too forward or "improper." In other words, letters are the things that help give romantic relationships a nudge in the right direction.
At the same time, however, if a steamy love note falls into the wrong hands (like when the Duke of Milan reads Valentine's letter to Silvia), a simple thing like a letter can shift the plot in an entirely new direction. In the case of the Duke reading Valentine's letter, the result is that Valentine and Silvia's plans to elope are foiled and Valentine is banished. (That's so much worse than your teacher reading your note aloud to the class, isn't it?)