Those who use this phrase usually cannot resist tacking on, “So get used to it.”  Now there’s a great thing to tell a kid who’s perceived an injustice in the world.  Why don’t you tell her that her puppy’s going to grow old and die, too?  Probably a horrible death.  What you are asking is that the child accept that life is unfair, to not even try to make it any different.  Give up.  Is it any wonder that children grow up so damned cynical when they complain to those people who might actually be able to make things better and the only response they get is, “Life sucks.  Get used to it”?  Sure, children can whine a lot.  And often their concept of what is fair is based solely on whether they like it or not, which is truly unrealistic.  But that takes time and patience to explain to a child.  And who wants to do that when it’s so much easier to use a cliché?