Romeo and Juliet begin and end their torrid love in darkness. When Romeo is heading to the Capulet's feast, it is in the nighttime and they are disguised in the darkness of a mask so no one will discover their identities. At the feast Romeo first touches Juliet's hand in darkness. They reach around a pillar till they find each other's hands.
Romeo climbs an orchard wall and is surrounded by darkness when he sees Juliet on the balcony and he is taken by her beauty and the way she leans her cheek upon her hand. Only when she is asking Romeo to deny his name does he answer. When they make their vows to each other he swears by the moon and Juliet says not to swear by the moon because it is not constant. The moon signals night time and darkness. This seems to foreshadow a dark and gloomy start to their love.
Even in the end, Romeo and Juliet die in the darkness of the catacombs surrounded by bones of ancestors and the darkness of death.
Reference: The Literature of Language Book by McDougal Littell
Romeo and Juliet are supposed to be enemies. They come from feuding families and they should hate one another. But they meet, and they are infatuated.
But in order for them to communicate, they must have privacy. Darkness provides privacy. Romeo seeks Juliet out at night, when he can sneak into the garden unseen and peer at her on her balcony. Because he is hidden in darkness, Juliet speaks freely of her love for him and her frustration at his true identity. When he speaks out, he is able to speak freely. He has the power of privacy and shadow to make him more bold and allow him to speak his feelings more freely.
Of course, the fact that they have to share their feelings in darkness foreshadows the tragic end they will come to.