Why do your nurses leave?

Just wondering what makes your coworkers finally leave. It's love/hate many times for me but I've only been in ER setting for a handful of months.
I did notice, many of my quiet frustrations ect are verbalized by many of my seasoned coworkers. I keep trying to tell myself the longer I'm there, it should be getting easier... When I hear it gets to them too, kind of hard to remind myself it will get better
Just curious about others input. I love it more than I have my moments, just trying to keep the big picture in mind.