Depending on the field, I think a lot of it has to do with the fact that once you enter the professional workforce for the first time out of school/college/whatever, you're going from an environment where everyone is at a roughly equal level of education, to one where some of your coworkers will have 20+ years of experience in the field. A new hire thinks that they're not doing as good of a job as the people who have more experience (which in fairness, they aren't), and that self-doubt sticks with you even as you progress through your career.
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Depending on the field, I think a lot of it has to do with the fact that once you enter the professional workforce for the first time out of school/college/whatever, you're going from an environment where everyone is at a roughly equal level of education, to one where some of your coworkers will have 20+ years of experience in the field. A new hire thinks that they're not doing as good of a job as the people who have more experience (which in fairness, they aren't), and that self-doubt sticks with you even as you progress through your career.