Sc. 1, Lines 1-11:why does Shakespeare open this scene with Romeo in a joyful mood?

In Act 5 Romeo is happy because he had a dream that he was with his love and became an emperor. Shakespeare starts Act 5 this way because it contrasts with the rest of the Act. His happiness at his dream of being awoken from death by his love is foreshadowing what will happen in the tomb later. It's this happiness at the dream that leads the audience to think that maybe Romeo will wake up when Juliet kisses him after he takes the poison.