Question

What motivates Teiresias to approach Creon and attempt to get him to see the error of his ways?

Answer 1

Creon's decree that Polyneices and other rebels should not be given a proper burial according to Greek religious custom is a direct violation of the will of the gods. Antigone is determined to follow the will of the gods (remember, she was born to a cursed family and knows the penalty the gods can exact for attempting to usurp their authority over mortals).
Teiresias is motivated to warn Creon because he "sees" that the gods are angered and are going to destroy him if he follows through with his arrogant and willful (see: hubris) course of action.