Mostly because of people. People can love a subject, love a job as it is supposed to be, and then when they actually work in a respective field, work with predominantly stupid, insubordinate, and distasteful people, and wind up doing all sorts of boring, stupid things and having stupid conversations for much of the day. One can see this commonly in education and healthcare, especially in so-called "inner cities". Let's mention the traffic-ridden commutes in the morning.
To anyone who found their "dream job", hats off! I'm damn grateful I have great health insurance, a salary, and some co-workers I actually like, and who like me in return. I'm convinced that's a lot to ask for in modern day America, if the current stat of 95 million able-bodied American workers sitting on the sidelines is true, and which I believe to be true.