Have you ever had to make a point to someone or a group? Chances are they will be able to relate to the point if you can compare it to a personal story that happened to you. Not a story that happened to someone else; but a story that you experienced.
People like to hear about your stories because they are relatable and they like to hear you suffered a little. No one wants to hear about all of the good news stories from you. They need to hear that you at least suffered a bit. It makes you more human.
Because you are attempting to make a point to the group you are addressing, the story must be relatable to the point. That is why preparing for the discussion is necessary and having an agenda is important. If you do not know what you are going to make a point about, how are you going to tell a personal story that is relatable? So prepare ahead of time.