What does this quote say about Santiago: "Then he will turn and swallow it, he thought. He did not say that because he knew that if you said a good thing it might not happen"?

On the eighty-fifth consecutive day without catching a fish, Santiago finally feels a massive marlin bite his line and begin to eat the sardines on the end of his hook. As Santiago carefully feels the marlin eating his bait, he becomes elated but manages to maintain his composure. He encourages the marlin to eat the bait as he desperately wishes for it to swallow the hook. Santiago is an experienced fisherman and is aware that the marlin currently has the hook sideways in its mouth and is swimming away with it. Hemingway describes Santiago's thoughts by writing,

Then he will turn and swallow it, he thought. He did not say that because he knew that if you said a good thing it might not happen.

Santiago's thoughts reflect his optimistic, cautious personality. He has faith that the fish will shallow the hook, yet refrains from audibly commenting on success for fear that it will not happen. Santiago's thoughts reveal that he is in tune with the natural environment and is a humble, experienced fisherman. He has also witnessed how ignorant, inexperienced fishermen have prematurely celebrated and lost their catch. Santiago does not want to jinx his good fortune and refrains from celebrating, out of fear that the fish will escape. His thoughts also reflect his cautious personality and desperate situation. He realizes that he has a rare opportunity and is able to maintain his composure in an exciting moment.

Santiago, the protagonist of Ernest Hemingway's The Old Man and the Sea, is an old fisherman who has never had much luck in life. Despite his track record of bad luck, Santiago is an optimistic person who doesn't let the circumstances in his life take away his passion for fishing.
The particular quote sums up Santiago's lingering fear of another misfortune. Although he is optimistic, he is aware of his propensity for bad luck, and his past experiences made him superstitious about "angering" fate.
The quote also shows the internal optimism Santiago holds. He keeps his words to himself for fear that his internal optimism will affect the outcome in the external world. This duality of internal versus external is prominent throughout the story. For instance, Santiago's battle with the fish is an internal victory, and shows that man's relationship with nature, with the external world, is experienced from within.