Okay, here's my thing...I think that the use of the phrase "life isn't fair" is kind of dismissive and teaches children to be complacent and take what is dealt. It also seems to be used most in a situation where one person is actively and presently being treated unfairly. With DD, she was accused by the lunch lady of doing something that she didn't do (please don't challenge me on this, I know she didn't do it, even her classroom teacher knows she didn't do it). DD insisted that she didn't do it, and pleaded her case to the lunch lady, who took it as "talking back." Basically, the teacher (who I otherwise adore) deemed that "sometimes life isn't fair" and punished DD, not for doing what the lunch lady said she did (which even the teacher knows she didn't do, remember), but because she argued and "talked back" instead of accepting the accusation, even though it wasn't a fair one.
Now, I totally realize that there comes a time in a person's life when wailing "that's unfair" no longer works, and we're expected to articulate ourselves better, but I'm not sure that squelching a child's natural obsession with fairness is a good thing to do. I mean, don't people "overly" concerned with fairness become labor organizers, human rights activists, and community organizers? Isn't there a place in society for them too? Does the "life isn't fair" mentality contribute, even a little bit, to people not even going to the ballot boxes to vote on issues that don't directly concern them? (Gay people can't get married? Life isn't fair! Poor people can't get free birth control? Well, life isn't fair!) I don't know if DD is overly concerned with fairness, but she certainly is deeply emotionally invested in it. And it generally isn't actually about her, and whether she is being treated fairly, but about the bigger picture. They are having a book swap at school, where kids bring in a used book and get to take home a new-to-them book in return. DD thinks it's unfair because not all children have books, or extra books, and that they should get to take home books too. So, she has resolved to bring in extras (I tried explaining to her that the school has already taken care of this, and that everyone will get to take home a book, but she doesn't believe me LOL). She also thinks it's unfair that polar bears are going to go extinct because we drive her to school (well, we walk now LOL, in all sorts of weather ). But when something unfair happens to her, she takes it very personally, not in a temper-tantrum-until-I-get-what-I-want way, but weeping inconsolably for hours, and her feelings remain wounded for days, or more. Literally. Yes, I try to teach her about picking her battles, but I just can't bring myself to tell her that life simply isn't fair, or to suffer anyone else saying it to her either. I am actually more upset about this than I expected, and we haven't even touched on the subject of "talking back" to an adult who is unfairly accusing you of something LOL! I just honestly cannot think of a situation where I would be so dismissive of another adult to say to them "sometimes life isn't fair!" I would never say something like, "sorry, DH, I spent $100 on xyz, now you have no spending money! Life's not fair, so suck it up!"