We are very unsatisfied with an action which is incomplete. A three line joke may tell a story, may describe an action, but unless it it large enough, we can't really get very interested in it. Because we were able to identify with one of the characters in the story in order to apply the lesson to our own lives. One test is whether or not all of it actually happened in the way described in the story. But that is only one test.
Some stories convey truth in other ways.
Sometimes stories that are absolutely factual convey ideas which are very misleading. That's because we generalize from specific examples. If a man is accused of rape and it turns out that the woman accusing him has simply concocted the charges in order to embarrass him, we cannot conclude from that that most men accused of rape are innocent. Just because the facts of the story are true it does not mean that we can draw conclusions from the story which are necessarily true. One of the reasons we like stories is because they allow us to take experience, manipulate it and make it controllable.