"The vanity of being known to be trusted with a secret is generally one of the chief motives to disclose it"

This quote by Samuel Johnson talks to the human tendency to want to be viewed as important and trustworthy. It suggests that when someone is trusted with a trick, they frequently feel a sense of vanity and pride in being trusted, and this can be a strong incentive for them to divulge the secret. This could be since they want to display their importance and reliability, or since they want to acquire recognition and admiration from others. It could also be due to the fact that they wish to feel consisted of and accepted by their peers. Whatever the reason, Johnson's quote suggests that the desire to be seen as essential and trustworthy can be an effective incentive for someone to expose a trick.