Why does Jocasta get excited when she hears about Polybus' death?

Jocasta's excitement upon hearing about the death of Polybus stems from her belief that this event disproves the prophecy concerning Oedipus. According to the prophecy, Oedipus was destined to kill his father and marry his mother. Since Polybus, the king of Corinth and Oedipus' supposed father, has died of natural causes without any involvement from Oedipus, Jocasta interprets this as evidence that the oracle's predictions are not to be trusted. This false reassurance leads her to feel hope that their past actions and fears regarding the prophecy may no longer hold any weight, signaling a turning point in their lives. Her excitement reflects her deep desire to dismiss the terror of fate and the implications of the prophecy that has haunted their family.