I hate it because I despise being judged without giving my testimony. People ask this question because they want to size you up. They're prejudging you before they actually judge you. Some people will argue, "No, people aren't judging you, they're just curious." It's possible that those asking might not be consciously aware that they're sizing you up, but that's exactly what they're doing. The answer to The Job Question will weigh heavily on whether or not anything else you say is interesting to them (regardless of whether it's actually interesting).
Before they get to, "What are your hobbies? What kind of music do you like? What kind of food do you like? Have you read this book?" they want to know if you're even worth listening to, which they will decide based on your job function. Rest assured, if you say, "I'm a cashier at Kroger," everything you say after that is significantly less interesting to them. Conversely, if you say. "I'm a lawyer," everything you say is significantly more interesting.
The content of conversation is irrelevant. You could immediately move on to a fascinating, well educated, well cited dialogue about the origin of the universe. People will think, "Well, this asshole works at Kroger, so what does she know?" Or, if their minds are a little more forgiving, they might think, "Wow, she's really cool and interesting....but she works at Kroger. Ugh."
In the opposite scenario, people will think, "Well, this fine, upstanding citizen is a lawyer, so she must be really smart and worth listening to." Even more offensively, you get a pass at being boring if you have a good job. Perhaps the next topic of conversation is the rate at which grass grows. A person is likely to think, "Wow, she's really boring.......but she's a lawyer, so she's smart and makes good money, so she's cool!"
I don't claim to be a mind reader, but I do claim to be a decent listener and observer. I listen to what people say about each other and I watch how people interact with each other. As is natural, I catalogue these patterns in my mental rolodex. I am also guilty of having passed the same judgments. Most of us have done it in the past and, sadly, will continue to do so in the future. There's no sense in denying it.
Go on clinging to your bad habit, defending it however you want. Go ahead, lecture me on some pseudo-scientific bullshit about "evolutionary psychology," and, "human nature," that you're trying to shoehorn into the discussion to make your behavior seem not only acceptable, but intellectually superior. At the end of it all, you know it's wrong. Why not admit the mistake and vow not to make it anymore? Trust me, it's a lot easier.
As a part of this vow, I think it's only fair to not want to be judged myself. So, I refuse to tell people what I do until I get to know them a bit better. I find, "I don't like to talk about work," to be pretty effective. It's funny to watch people shockingly scramble to find the next topic. Some don't care and move right along, some are taken aback a little, but move along, and some of them are actually offended. How dare you not let them judge you by your job function?!
You are you. You are not your job, especially in this crappy economy. At this point, anybody who's successfully supporting herself is worth admiration, even if she's cleaning toilets or working at McDonald's or some other job people negatively scoff at and judge by. If you work hard and pay your bills, you deserve pride in yourself and respect from others.
It's shameful to judge someone on how they earn money. It's shameful to look down on someone because you think they have a "crappy" job. It's shameful to admire someone just because you think they have a "great" job. You're being a classist cunt.
So, don't ask, don't tell.
Image stolen from icanhascheezburger.com, who may have actually made it themselves.