Ideally, teaching and learning environment should be comfortable and resourceful with curious, enthusiastic and excited participants with high expectations.
Excitement is a sign of caring and building motivation.
The best way to spread excitement to the learners is by the teacher being excited about the subject himself or herself. It is much better for students if they could turn for help to both, instructor and TA, and often they feel more comfortable to discuss their questions with TA first. A number of our TAs are not excited about the assigned subjects, often are behind the teaching process and are reduced to passive graders and menial supporting staff. A TA, who does not know subject well, sets a bad example to students about our professionalism and educational values. If a TA is not up to the job, it does not only increase the instructor's load, but more importantly, it will lower students' morale and frustration of all involved.