Have just come home from a session with my trainee teacher,and,as after each session with him, I thought a lot about how much I discover I have learned as a teacher during my 29 years.It was amusing to read this New York Times article  about what makes a good teacher, and how people making a career move into teaching in mid career often drop out quickly ,despite their previous job experience. We (teachers) all know that NOTHING prepares you for life at the “chalkface”.
I am not sure that I agree with the statement that you either love it or you don’t, because I frequently find myself both loving and hating it simultaneously on the same day in different classes, or in different situations.
However, there is no doubt that no training course can prepare for the shock of 40 pupils all trying to get your attention at the same time, or alternatively, ignoring you totally.
The tiredness one feels after 7 straight classroom hours is like no other that I know, and that doesn’t improve after 29 years.
Add to that the fact that the majority of teachers on the teacher training courses have not set foot in a high school classroom for some years, and it is clear that you can only learn how to teach by just doing it.