Each time I get feedback from clients on what a difference I've made in their lives, I'm amazed. All I do is listen to my clients, address their needs, provide them with fun and effective workouts and do my best to convince them to eat well and exercise.
Doesn't sound exceptional to me, but it is. Few people have others who care enough about their health to act, so you often are the client's only lifeline to a healthier, happier existence.
Make sure you always show up - mentally as well as physically. Listen, and really hear what they are saying. Let them know you care because the impact you have in your clients' lives will ripple through their futures.