What transforms the balcony scene in Romeo and Juliet into a moment of great poignancy?

The balcony scene, one of the most beautiful in all of Shakespeare's plays,  is especially poignant because the audience knows from the Prologue that the love between these two young people is doomed, and that it will consume the two of them. They are "star cross'd lovers" who are destined to "take their life" due to their love. So in this scene, where the two lovers declare their love and devotion to each other, even planning to marry, the audience knows that it won't work out. Even Romeo and Juliet themselves are conscious of the realities of their relationship. Romeo says that his name is "hateful to myself" because it makes him Juliet's enemy. Juliet, before realizing that Romeo is listening in the garden, hopes that he can "deny his father" or that perhaps she can disavow her own name. The audience knows that neither of them can actually deny who they are, however, and their love will be consumed by the rivalry between their two feuding families. This is why this scene, full of the most tender expressions of love, is so sad and powerful.