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Huge resentment issues in both those lines of work that are often a projection of some other sentiments the worker might have. At least that's what I saw. People fed up their lot in life so taking it out in other ways. Or really fed up with the job itself/mistreated/underpaid so anything they had to do was seen as a massive ballache to them. And because the general public are seen as the ones asking them to do this work, they naturally got branded as the arseholes. These feelings then only got reaffirmed when they had to deal with actual arseholes.
I'm not sure this is true really, but I kind of understand the sentiment.