I have always reacted somehow uncomfortably when someone respond to your initial thanking comment with a professional, distant and formal «This is my job, sir» or even worse for me, when the answer is still more distant (less often I must say) «This is part of my job, sir».
What do we think we are at a a job for? Naturally (at least in many jobs) to serve, facilitate or help others (consumers, customers or simply colleagues, when we are part os a process within a company). Anyway I do know few people that expect a clearcut result, a simple outcome as a consequence of an interaction process.
As a matter of fact, most of the times what we get is the mere interaction process, regardless of what we get (the good, the service, etc.). And this is what makes more relevant for us the very same interaction, I mean we recall the experience, we keep a good image of the company, supplier, attendant… and we extend our image for a next opportunity. Therefore, I do not care much «What is your job», but «How you actually do your job with me», and this is the reason why you are there, providing me (or whoever).
So please do not consider someone professional if he/she is not «creating» his/her job. We do not need a machine delivering the expected outcome, but a human being that put his/her best knowledge, attitude and expectation on daily job. This will shape our intention to return to this office, servant, point of sale,… for the next opportunity.