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Being told what to draw, endless revisions, tired subject matter, getting harsh critiques, it can be a downer.
Ways to keep it fun for yourself is to try to inject a little of your personal tastes into it, or figuring out ways to also make the work an opportunity to practice some technical aspect of art. There's also just working on techniques to up your efficiency, so even if you're bored at least you're belting it out.
Also- if you're that bored and on autopilot, it may be a sign you're ready to advance to more technically challenging work. If none is available, just enjoy listening to podcasts until you can score those kinds of jobs.
Finally- never show proficiency at something you hate unless you're trying to score your first few jobs. Show off in your portfolio the kind of work you want to do most, esp. if you've been in the industry for awhile.