But that was three years ago. Today, I think I've got it. They leave because they are not welcomed, not valued, by their schools. They are being asked to make bricks without straw by tired leadership teams following the path of least resistance.
Last year, two-and-a-half years into my career, I joined a local school. I was offered A-level teaching, with responsibility points. On the surface, it seemed like a great job. Looking back, there were tell-tale signs that should have made me think twice. There was no work on display in the rooms or along the corridors, the students seemed depressed and the atmosphere in the staffroom was grim. No one seemed to recognise the head.
But I wasn't looking for a perfect job, just a good one, so I threw myself into it and gave it everything I'd got. I worked hard, pulling round some very difficult, demotivated classes. I organised a couple of major school events and theatre trips and poetry days. I coached sports teams, started a drama club, helped with the school play. And the more I did, the more obvious it became that there was something wrong, right at the very heart of the school. The head and the senior management team were holed up in their offices, effectively removed from contact with students, parents and staff, issuing an endless stream of memos. The school was fine on paper, but on the ground difficulties were ignored and staff were left to sink or swim. But I was enthusiastic, new to the profession. I wanted to do my best for the students and I felt there was something at the school I could build on.
Then my new timetable arrived. It just appeared in my pigeonhole on the last day of term. My new year was going to be made up of 53 lessons across two weeks. Eleven teaching groups in 17 different rooms; only seven free periods. I spent eight weeks up to the October half-term trying to make it work. I was working without a base, without a cupboard or a desk, running from room to room and doing my best to deliver good lessons and progress my students' learning. I didn't want to complain but I was in an impossible situation, and so were my students. I had to ask for help. I went to my line manager and I went to the head, explained the situation and offered positive suggestions. I asked for their support.
What I got back was sympathy, soothing words, praise for what I had managed to achieve under what, they conceded, were very difficult circumstances.
They promised future meetings and attempts to help.
That was in October. Four months later there have been no meetings, no changes, no support. I'm still working from the boot of my car. My students are still forced to roam the school from hut to maths room to language lab.
Perhaps I've been unlucky. My first school wasn't like this. It lived up to its mission statement. So maybe my current one is the exception. But I'm not so sure. Many friends who qualified with me have similar stories. We work in schools where we do not feel supported, where management teams avoid difficult issues, where senior staff hide behind paper barricades and promise "jam tomorrow". And however much we love the job, when we look into the future, too many of us - full of hope and enthusiasm just three years ago - feel uncertain and unconvinced.
The writer, who wants to remain anonymous, lives in the north of England