And the easter bunny isn't real, either
On more than one occasion, I've said something to another person that wasn't considered to be mean-spirited in the least. It's all about the tone, and mine is sarcastic (especially after one or two drinks). Usually, the truth entails me saying something along the lines of "You do know they're not interested in you because they're dating someone else."
What I'd think would be common knowledge comes across as a shocking revelation. "WHAT?! How you could you say that! Of course he's madly in love with meeee! You're a monster! Everyone knows he's just waiting for the right time to be with meeee." And, the rant continues until they're in a sobbing mess, lying on the ground, heaving from hyperventilating.
It's like telling a little child the easter bunny isn't real. The only thing that's different is this particular child is an adult (at least in age). You're bursting their bubble without knowing it, but for chrissake, someone has to.