What causes romeos mood to change at the beginning of act 5?

At the beginning of Act 5, Romeo experiences a sudden and dramatic change in his mood from joy to despair upon learning that Juliet has seemingly taken her own life. Balthasar, Romeo's loyal servant, arrives in Mantua and informs Romeo that Juliet is dead. Romeo's initial reaction to this news is disbelief, followed by overwhelming sadness and grief. His mood shifts from the hopeful anticipation of reuniting with Juliet to one of profound despair and sorrow.