...I do it because it's my job, not because I am trying to make you happy. Don't get me wrong, it is good to be appreciated but thanking me in this way puts you in a position of authority that you may not actually have.

Conversely, I won't thank you for simply doing your job, unless it was part of some truly kind action for which I am likely to be genuinely grateful. I might compliment you on doing a good job, sure. But I won't thank you for it.