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X complains about their job. Y says "fuckers should try my previous job", which implies "X has no right to complain because they have it easier than I had".

By that logic, Y has no right to make this complaint if anyone who has worse job than Y exists. They do exist, therefore Y has no right to complain about X's complaining.

Y's position is inconsistent. It says "You have no right to complain because your job isn't hard enough, but I have a right to complain because my job is/was hard enough".

It establishes an arbitrary baseline of having-a-difficult-life and consequent implied manliness/human worth that Y is above, but X is below. It's a way to smugly look down on someone for not being as awesome as you are, while trying to project a humble "I did a shitty job" image.