Z-M Where to begin!
The easy one to explain is that it is physically exhausting, especially if you are in a large school and carting bundles of books and materials around all day.
Also, you are often being challenged, sometimes physically assaulted (it's happened to me) you are coping with 100s of teenagers with raging hormones, personal and social issues and you are trying to establish a calm, achieving environment where your lessons (which have taken a lot of brainwork and must always be ready to adjust to events and queries) can be absorbed. Then in the evenings, you cook dinner and ignore your own kids for a bit (or if you don't, you start marking at 9 or 10 p.m) and mark homework, sometimes say 60/90 books. (My subject, English, canot be marked with a tick, there are spelling and grammar errors, etc. to correct.
I survived and was a good teacher, but it is by far from an easy job. Anyone who thinks it is, just try it. My daughter now teaches Maths. she has marking time built into the school day, but it's not enough and I would not like to have to fill in the forms for lesson prep. that she does. Justifying every approach is just the start.