I witness both kinds of dialogue in the store each day, and I can tell you that people who don't think about what they say often hurt someone's feelings or ruin a person's day without even realizing it. So I've created my list of suggestions not to do, especially in public where everyone else can see a hurt face, even if the offender is oblivious.
1. One-upping: I know, I know. We all do this occasionally without even thinking about it. Your friend is telling you about her terrible toothache and you interrupt with, "That's NOTHING! I once had a root canal and dry socket all on the same day!" Or, as my aunt put it when admitting to one-upping her mom's bad day, "Dead husband trumps bad pancakes anytime." It's hard not to chime in with a similar experience sometimes, but all your friend (or customer or whoever) really wanted was a listening ear and someone to sympathize with her. So keep the fact that you had cancer and multiple sclerosis to yourself when someone mentions struggling with chemotherapy (note: this actually happened).
2. Use the same accent/speaking style: Let me explain this one. It's courteous to not use big words when you're speaking to someone with a limited vocabulary, such as a child or a person who isn't educated or is unfamiliar with the language. It is NOT courteous to speak broken English to someone who speaks broken English to you or to talk baby talk to the cashier taking your money. Speaking this way is not only demeaning to the person you're speaking to, it makes them feel as though you're dumbing yourself down, or that they're too dumb to understand normal speech. Just speak as you normally do, and leave the "Me no good at speaking" to those who are still learning their grammar.
3. Ask people blunt, open-ended questions: It's one thing to joke around with someone you know really well who KNOWS you are joking with such questions as, "Why do you still have that ugly haircut?" It's another thing entirely to ask someone a question without a simple answer, such as "Why aren't you serving a mission?" or "Why aren't you married yet?" The best thing to do is to ask, much more tactfully, "What are your plans right now?" If they're comfortable telling you that they scare off potential husbands/wives or that they'll only marry someone who's got the money, then that's their prerogative.
4. Tell people they look better when they change their appearance: This wouldn't seem like something that could hurt a person's feelings, but think about what it's implying. If you gush about how great someone looks now that they've lost 20 pounds, cut their hair, or dyed it, they start to think, "Hmmm, why is everyone suddenly paying attention to me now?" or "I thought I still looked pretty good before I went brunette." Be tactful when you tell someone they look nice, especially if their change in appearance is something temporary, like straightening their hair. They might just be experimenting.
So, I didn't make it to five or even to ten suggestions for being a bit more tactful. But maybe you recognized someone saying one of these "don'ts" to you, or possibly realized that you do these things yourself sometimes. All I'm saying is that it makes life at a grocery store (or wherever you work) a little better when everybody plays nice. Here's hoping I'll play my cards more wisely tomorrow. After all, "nice haircut" should really trump "still single" anytime. In my deck, that is.