It is a very common trope in every genera from Arthurian romance to hard science for secrets to be kept. One character keeps a secret from another. A government keeps a secret from the populous. Most of the cast keeps a secret from an individual An secret is kept across ages due to people simply forgetting. . A secret is revealed in moments because a character can't stop talking. Secret keepers are portrayed as honorable, horrible, dishonest, just trying, or completely in command.
But usually, keeping a secret involves some dishonesty and dishonor. There is some stain on the secret keeper's soul even if they have the best of intentions.
So here is a question. What is the best reason to keep a secret? What motivation provides the most honorable position for the character?