I was a career changer into teaching, from financial services. Worked in the City, long days and commutes etc.
Teaching is stressful. It's mostly a lack of control thing. Teachers can't always control what and how they teach, how they mark and feedback etc etc as well as that whilst you have control over classes it's....a fine balance. It doesn't take much to too things.
That can lead to feelings of stress, as can any job where the incumbent isn't entirely in control of their work. Add on the fact you can't always help kids you desperately want/need too, parents who call you an effing c for giving their child detention, constant monitoring by management and a never empty inbox and it's a toxic mix of feeling not good enough to do the job and task overwhelm.
Maybe it's because teacher are overwhelmingly helpful types. They want to help, want to do the best they can. They go in with the very best intentions to make a difference and there's not enough hours or money to achieve what they feel they ought to, for the kids. To do the job they want to do. Again, lack of control, feeling not good enough. Stress results.
By no means is teacher stress 'worse' than any other jobs but it is hard to get those outside teaching to see that it's not a 9-3, 37 week year.
I LOVE my job, but in part time and work in a super school with sensible policies and a commonsense boss. I've worked in schools where it's not like that and it's wretched.
Its like any job, you've got to find the right place and people, problem is that the burnout rate is huge. Most new teachers dont last five years. That's a huge issue.