When the instructor has enthusiasm in the classroom, students can feel more at ease and be more inclind to interact and retain what is being said. It will also allow students to interject more and not be afraid if the answer or comment is not correct when the instructor corrects them. also like Daisy stated if they are having a bad day or came from a class they did poorly in coming to your class lifts them up because they know they will be made to feel good about themselves.
Christina