The tone of Romeo and Juliet moves back and forth between the light-hearted/comic and the intense/tragic. We can appreciate that Shakespeare provides a Prologue that tells his audience this will be a tragedy, because the tone is so often comic, especially in the beginning scenes.
Comedy includes Romeo's over-the-top mooning over Rosaline, the only woman, he claims initially, that he could ever possibly hope to love, only to drop her instantly the moment he sets eyes on Juliet at the Capulet party.
Mercutio, with his wit, puns, and strong storytelling skills, as well as his lively personality, also contributes to the lighthearted tone of the play. Juliet's nurse provides comic touches, too, in her outrage at Mercutio's jokes at her expense and when she takes forever to tell the news of the marriage plans to the anxious Juliet.
Alternating with these, however, are scenes of intensity and anguish, such as when Tybalt kills Mercutio and Romeo kills Tybalt. The intense emotions aroused by those events continues with Juliet's deep distress at Tybalt's death and in Romeo's banishment. As the play comes to a close, the mood also darkens—Juliet alone in her ancestors' tomb has a deep sense of foreboding, and the suicide scenes, of course, are filled with pain and anguish.
Shakespeare keeps the play alive and engaging with his interplay of the lighthearted and tragic tones.