What motivates odysseus to reveal his true name to the cyclops?

Pride. Odysseus had just blinded a Cyclops, who otherwise would have killed him and his men. He wanted the Cyclops to know who it was that had beaten him, and so he revealed his name to him. It would later turn out to be a bad decision on his end. 

Pride motivated Odysseus to announce himself. He wanted his heroics to be remembered.