Dear Coworker

If this really were your dream job, as you claim, I would think you'd want to do a better job at it.

Here's a clue to help you do a better job at your job:

Do your job to begin with.

That way, I wouldn't have to do your job, since your job needs to be done before I can do my job.  And quite frankly, I'm getting tired of having to do your job as well as mine.

Thank you.

That is all.