Yo get the mental satisfaction by affecting the life of someone who is in need. Your work have social implications and improve the social welfare of people, which is making you a global change agent. You are collaborating to change the world by giving hope when people feel hopeless.
Seeing any sign of progress in the persons quality of life in the positive, than it was before you met! Some measure progress in large sometimes unattainable amounts! If, for instance the person came to you at first meeting, and would not look you in the eye for many sessions, then one day they came in and maintained eye contact the entire session, that is progress and it gives you a sense of accomplishment and pride! You may not reach every one you work with but if you can reach one, you’ve jumped the moon!