Why did Creon side with Eteocles and reject burial for Polynices?
After Oedipus' blinding and exile from Thebes, Creon became the king until Oedipus' sons became old enough to rule. Because both Eteocles and Polynices seem to have been legitimate heirs to the throne, a solution was worked out whereby they would take turns ruling Thebes. One would rule for a year, while the other went into exile.
Unfortunately, Eteocles, after a year as king, did not want to give up the kingship. Polynices wanted his own turn at being king and so he raised an army from Argos (where he had spent a year in exile) and marched upon his native land of Thebes to try to regain the kingdom by force. In the ensuing battle, both Eteocles and Polynices died, killing each other simultaneously.
The reason Creon wanted Eteocles to have a proper burial was because he died defending his country. Because Polynices had fought against the Thebans, Creon declared that Polynices should not be buried.