So we have scripts for our social interactions. You don't actually tell a passing acquaintance that you're having a shitty day because your car broke down and you had a fight with your girlfriend, because when they asked how you were, they didn't really want to know: they're just following the script. The courteous thing for you to do is to ease the interaction by also following the script. If you feel bad about this, think about what question is really being asked.
I find this comes up most often with checkout workers in stores, who are on a certain kind of autopilot. Take the cashiers at Whole Foods. When I go there to get a breakfast burrito in a little takeout box, they ask, "Is that for here, or to go?". It's to go, but if I tell them that, they put a piece of tape on the box, which I neither need nor want. Strict honesty in this case, saying "It's to go, but please don't tape it up", slows down the transaction by many seconds. It interrupts the Cashier Autopilot, and for what? It's rude. The question they're really asking is, "Do you want tape on that?". And the way to answer "No" is to say "It's for here." I do similar things with places that want my ZIP code or phone number for their marketing databases: the cashier doesn't care about my privacy concerns. We're both happier if I give them a fake ZIP code, or if my phone number happens to be 555-1212 (sometimes they notice that, which is also amusing).
I am *stunned* at the quality of the latest round of best-of-craigslist. We have Brazilian waxing (by a gifted Vietnamese woman), the upsides of a cheating husband, and naked hot tub neighbors. This is really a bumper crop.
One of my cow-orkers, in QA, is a nice guy a couple years older than me, from outside Boston. We got off the train this morning, and it was chilly and drizzling; as he zipped up his jacket, he said "When did I move to Seattle?", and commenced a session of typically Californian bitching about the rain with a couple other colleagues. I finally looked at him and said, "Don't be such a fucking pussy". He cracked up. I like my job.