In Matthew 13:10-17, Jesus explains the purpose of parables. It is not to expand the meaning, but to hide the meaning from the people whom He did not want to understand. Only with the Spirit of God can we really understand the parables, but He has to give us the understanding. Certain keys unlock parables, and if we do not have the keys, we will miss the meaning, and the interpretation will be wrong, off track.
Other parts of the Bible tell us that it takes the Holy Spirit to give us "ears to hear" (see I Corinthians 2:6-14). To understand spiritual things, we must have the Spirit of God in our minds - we must have the mind of Christ - that in turn opens up the Bible's "mysteries." Of course, the other major key is the Book itself, because the interpretation of the parables is within the Bible. It is not necessarily just the scriptural context of the parable; rather, the entire Bible contributes to opening up the parable's meaning. In other words, a parable does not stand alone. It must "fit" within the Bible's revelation.
Richard T. Ritenbaugh
Parables of Matthew 13 (Part 1): The Mustard Seed