Tone is one literary device Shakespeare uses to clearly portray Romeo and Juliet as a tragedy. Tone should not be confused with mood. Mood describes the atmosphere of a piece of literature or the atmosphere of a certain scene or part; it describes the characters' feelings. Tone, on the other hand, describes the author's attitude toward the piece. Shakespeare generally paints a very disapproving tone towards the events of the story, which not only helps relay his important themes, but also helps to paint the play as a tragedy.
It's quite evident in several places that Shakespeare not only disapproves of the actions between the Capulets and Montagues but also of Romeo's and Juliet's actions as well. We first hear Shakespeare's tone, or attitude, towards the subject in Prince Escalus's speech in the very first scene, especially when he refers to the warring families as "[p]rofaners of this neighbour-stained steel," which is to point out that both families are misusing their swords by attacking their own neighbors rather than their enemies (I.i.78). We even hear Shakespeare's disapproving tone in the voice of Friar Laurence, who, when we first meet him, points out that, like a poisonous flower, man has a dichotomous nature as well, which is "grace and rude will," and that when man allows his negative characteristics to be the more dominant ones, "[f]ull soon the canker death eats up [mankind]" (II.iii.28-30). Even Juliet acts, at first, as the voice of reason, allowing us to hear Shakespeare's disapproving tone towards the actions of the lovers. She acts as the voice of reason when she tells Romeo that, although she cares for him, she does not believe they should be making any promises tonight as it is "too rash, too unadvis'd, too sudden" (II.ii.124).
Shakespeare's disapproving tone serves to relate the dominant themes of man's character weaknesses and also of the consequences of acting upon violent, passionate, emotions rather than rational thought. Not only do the warring family members allow their violent, passionate emotions to govern their behavior, so do Romeo and Juliet. And, as we see, acting upon violent, rash, passionate emotions leads to death. Therefore, Shakespeare's literary device of tone is one major element that helps characterize the play as a tragedy.