An important part of listening is being aware that your body gestures can communicate as much or more than your words. Gestures begin with your lips. What are they saying to the speaker? A scowl may come because you are deep in thought about the next part of the lesson, but it will say to the speaker that you are not happy with what he is saying. Learn to enjoy and feel a sense of success in the process of the discussion, and you will likely maintain a smile.
If you are sitting, make sure your posture communicates interest in what the person is saying. Sometimes hands behind the head, or leaning back in your chair, suggest you are not engaged with the speaker.
Does your body language communicate that you are relaxed or tense? What are you doing with your hands? Are you grinding your teeth? Does your body language communicate that you can't wait for the person to finish so you can start talking again?
Bottom line – RELAX! Your body language will communicate whether you have or not!