Why do they do this? Maybe they are just poor communicators who expect people to read minds. More often, this is a "teaser" statement designed to pique your interest in a boring story about their lives you would normally ignore. Some people will fall into their trap and ask, "What's wrong?" and the poster can dump his or her story on those people without feeling guilty because, well, they asked!
Did you ever have to do book reports in grade school? Remember how you were never supposed to tell people how it ended? "Will the Hardy Boys manage to find the criminal before their friends are dissolved in a vat of acid? Read the book and find out!"
By the time we grow up, we learn it's not good to be that transparent and say, "How did my attempt to help someone backfire on me today? Send me a sympathetic message and find out!" so we layer it under a bit more subtlety.
Another from Failbook.
You could just be honest and say, "I had a crappy day and need someone to talk to. Message me if you've got some time." But this approach fell flat in our focus groups, far behind more proven marketing techniques like offering to enter the listener into a sweepstakes for every minute they listen to you, or promising a surprise guest, or just vaguebooking.