“Regard him as loyal, and you will make him loyal.”
Seneca suggests that suspicion can become self-reinforcing: treating a chosen friend as trustworthy helps create the conditions for candid, reciprocal friendship rather than teaching defensiveness from the outset.
Source details
Moral Letters to Lucilius, Letter 3, Richard M. Gummere translation. Seneca immediately contrasts this with people who fear deception so strongly that their suspicions help teach others to deceive them.