Juliet is impatient for night to come because Romeo, her new husband, cannot come to her until night. During the day, it seems, there is too much of a chance that he will be seen and they will be found out. So, on one level, she is simply anxious to be with him because she loves him.
Juliet is also very much looking forward to consummating her marriage with Romeo; in other words, she is anticipating having sex with him. She says, in part, "O, I have bought the mansion of a love / But not possessed it, and, though I am sold, / Not yet enjoyed" (3.2.28-30). She is eager to learn to submit to her husband, to become fully his, and to learn how to be a wife in the carnal sense. This isn't just about lust, though. She sort of sweetly wants to lose her virginity in a way that is proper, for her, still modest and still innocent.