It is interesting the concept of parable. In a sense a parable is a riddle about oneself or mankind.
In another sense a parable is the plainest form of communication about oneself possible at times.
It is always because there are some things which we must learn for ourselves. Sometimes we can only be guided to learn for ourselves because we will not accept plain speech.
Parables are plain speech because they seek to bring the truth into the light often in the most open form that could have any effect.