I've always seen it as akin to stage fright - I think someone else has commented on this. It's what you get if it matters to you that you do a good job. Towards the end of my teaching career (at a university) I got charge of the allegedly difficult classes - 3 intakes a year, ~200 students each time, all having arrived via some non standard route. My job was to get them up to scratch in 10 weeks. It was the most rewarding teaching I ever did. Very hard work, every class a challenge. If I'd been less experienced when I'd taken it on, I'd have drowned, but it was a perfect job for me at that stage. Nurture those nerves, they're a key part of the role.