It is important to note that Romeo begins the scene in high hopes. He has had dreams that "presage some joyful news at hand." He goes on to detail one particular dream:
I dreamt my lady came and found me dead...
And breath'd such life with kisses in my lips
That I reviv'd and was an emperor.
So definitely, he is cheerful and optimistic as the scene opens. But once he hears from Balthasar that Juliet is dead, he is so distraught that he buys an illegal poison from a passing apothecary, determined on his course of action -- "Well, Juliet, I will lie with thee tonight" -- to see Juliet one last time and then kill himself.
It should be noted that even though Romeo discounts his dream ("Then I defy you, stars!"), it was actually a positive omen and one he should have paid closer attention to. The dream told him that his lady was alive and that she would kiss his dead lips. These events will take place in the Capulet's tomb, once Juliet wakes, just as foretold in the dream. Unfortunately, Romeo doesn't recognize the omen that tells him Juliet is actually still alive, and so the tragedy will unfold as promised by the Chorus in the Prologue at the beginning of the play.