Why do Eteocles and Polyneices fight each other in Seven Against Thebes?

In Aeschylus’s tragedy Seven Against Thebes, the conflict between Eteocles and Polyneices arises from a dispute over the succession to the Theban throne following the fall of their father, Oedipus.1

The brothers initially agreed to rule Thebes in alternate years. However, after Eteocles served his first term, he refused to relinquish power to Polyneices. In response, Polyneices withdrew to Argos, secured the support of King Adrastus, and mobilized an army to reclaim his right to the throne by force. This power struggle ultimately led to the brothers killing each other during the assault on the city's gates, a tragic outcome that fulfilled the curse their father had placed upon them.12