No need to explain this one. Just don't do it. Okay, here's the explanation: if you have nothing but bad things to say about your last partner, your date might start to wonder about the other side of the story and possibly project themselves into a future conversation where you complain about them. If you have nothing but good things to say, it might look like you're still emotionally invested in your last relationship and not ready to move forward. Lose-lose.
3. Your Ex