In Book 12, Circe gives Odysseus information that is meant to help him on his journey back to Ithaca. Circe tells Odysseus that he will have to pass a monster named Scylla, who has six heads and eats humans. Circe warns Odysseus not to fight against him, but Odysseus decides to try anyway. He also decides not to tell his crew about this.
I believe that Odysseus was most likely right in not telling his men about the monster. If he had told the men, it is very likely that they would not have wanted to continue the journey, or that they would have been terrified while doing so. By not saying it, Odysseus is being a good leader and trying to get as many men as possible back to Ithaca safely.