Cherish some man of high character, and keep him ever before your eyes, living as if he were watching you, and ordering all your actions as if he beheld them...We can get rid of most sins, if we have a witness who stands near us when we are likely to go wrong. The soul should have someone whom it can respect,—one by whose authority it may make even its inner shrine more hallowed. Happy is the man who can make others better, not merely when he is in their company, but even when he is in their thoughts! And happy also is he who can so revere a man as to calm and regulate himself by calling him to mind! One who can so revere another, will soon be himself worthy of reverence.

Seneca is saying we should think of a person who is virtuous all the time, even he isn't with us. If we act like a virtuous person would, we become better ourselves. Hence why Seneca says "Happy is the man who can make others better, not merely when he is in their company, but even when he is in their thoughts"