This week's hypothesis is that great parables are bad stories.
More accurately, great parables tend to be incomplete stories, because one thing that makes a parable great is that the listener has to put himself into the parable. This is one way a parable differs from a myth; a myth tells you what your role is, a parable tells you what roles are available and challenges you to choose the correct one.
Think, for example, of the Parable of the Prodigal Son. It's one of the longest, most complex, and richest of the written parables of Jesus. Yet with both sons, the father has the last word. We aren't told how the younger son responds to having a ring on his finger again, nor how the older son responds to being told all his father has is his.
I was going to end the last sentence with "how the older son responds to learning all his father has is his," but, really, we don't even know that he learned it, we only know that he should have learned it. Since he should have learned it through the years of living with his father, we don't know that he learned it even when he was told directly. (An echo, perhaps, of Dives and Lazarus; can the older brother believe in his father's love when someone returns from the dead to show him?)
In short, we know how the brothers should respond, but we aren't told how they did. The parable leaves it up to us to say, "Today I am the younger brother," or, "I am acting like the older brother," and then respond to our Father as we should.
This idea of incompleteness may also help explain why so many who listened to Jesus preach did not hear what the parables were saying. We come to these parables with thousands of years of experience in thinking about how they should be completed; coming to it fresh, it's easy to be confused by all the ways they could be completed.