Workload and behaviour are the main drivers, but pay is also becoming more of an issue.
Someone I know has summed it up quite nicely when they pointed at the staff car park the other week and said, just look at what our teachers are driving these days. Many cars were 13 years old or older, often very old bangers that are lucky to pass their MOTs every year. I drive one such car and at the last MOT was advised to now just drive it until it falls apart instead of attempting repairs, but I also don't have the money for a replacement when that happens. I'm an experienced teacher paid on the leadership scale. One of my colleagues lives at home aged 25 because they can't afford to live on their own on an ECT salary.
As for workload, I have a friend in Denmark who was shocked that I'd be working this holiday. I told him that, despite being incredibly organised, I was unable to finish all the work assigned to me. He asked about overtime payment and was, again, shocked that no such thing existed. He knows I work 11 hour days most days, with some as long as 14 hours during term time.
Behaviour: when you have to watch every word out of your mouth while kids get away with being rude, insulting you, openly telling you they're stalking you and making tik toks of you, when parents complain about every negative point (not even detentions - almost every one of those gets questioned), when you have to justify everything instead of just being trusted to do your job and a bad class in an observation can break your career, then there is little motivation to stay.
Money plays a massive role, from not having glue sticks, a budget for practical resources, money for cover staff so every free lessons means being pulled for cover, no money to repair buildings (one recent storm took the roof tiles off and now it leaks), no money for tech so the kids don't engage - well, what do we expect? Kids don't even bring pens anymore and now we don't have the money to replace those, and kids are gleefully telling us they won't do the work, then.
It's broken.
Unless education is being taken seriously, and fast, by parents, children and politicians alike, this is a ship that will crash into the iceberg soon.