Another issue may be the potentially terrifying number of mistakes one makes when teaching a class of 20 or so young people who are less than eager to be in the room to begin with. 20 years in front of the classroom, and I still approach each day with varying degrees of dread of the unexpected. And should one have one of those ‘bad’ classes that never seem to settle, every day involves at the least a kind of clenching upon approach. A heavily-scripted lesson plan can go a long way towards soothing some anxiety. And even if the lesson goes awry (as it will inevitably do at least once), one can still close the period with the knowledge that you were prepared.
I can think of few jobs that involve so many opportunities for emotionally-charged mistake, and even fewer with less built-in mechanisms to confront them.