A parable, I have been taught since childhood, is a "heavenly story with an earthly meaning," which is good as far as it goes. The word parable, however, carries with it the idea of placing alongside. What is placed alongside what? you may ask. The answer is:
Our lives are placed alongside the story, and the point of the story is meant to stir something deep within our spirits, which causes us to align (or re-align) our lives with the point or moral of the story.
Think of the process of analogy, which can function as an extended simile or metaphor. Take the following sentence, for example:
With the president's arrival there was a sudden flurry of activity.
The implied simile is
The president's arrival was like, or resembled, a flurry of snow.
The picture this simile triggers in the minds of the readers (or hearers) is perhaps of a million things going on at the same time, with no small amount of confusion, scurrying about, nervous chatter, mild panic, and so on. The "analogs" between snow flurries and human activity in the face of the imminent arrival of a dignitary are many, but they are largely taken for granted by both writer and reader alike. One strives, however, for aptness and meaningful mental comparisons which enlighten and inform, not perplex and confuse. (For example, to say "With the president's arrival there was a mudslide of activity" would not be apt, not even remotely--I think!)
Now think of a parable as a simile, but a simile which is pared down so that the focus is not on many points of comparison, or analogs, but only one. This single point, then, becomes the basis for comparison between the story the storyteller tells and the life of the listener who listens (or reads).
Aesop's fables illustrate the modus operandi of parables pretty well. Take the story of the turtle and the hare, and the race between the two. The moral of the story, or the application of the story to our lives is
Slow and steady wins the race.
Did Aesop mean for his readers to take the story apart and start asking questions such as
What does the hard shell of the turtle represent?
What do the long ears of the rabbit signify?
To what, specifically, should we link both slowness and quickness?
Is it always wise to be slow and prodding in our behavior, or is there a time to be slow and deliberate and a time to be quick and intuitive or spontaneous?
How long was the race?
Is the race a symbol of two perspectives on life: stop and smell the roses, on the one hand, and keep pushing forward to the finish line of retirement and then stop and smell the roses?
Of course not. Neither, I suggest, did Jesus. Jesus had a point to make, a moral toward which his parables pointed, and then he expected his audience to lay their lives, so to speak, next to the moral of the story and determine for themselves (with God's help, of course) how and where, specifically, that moral applies to their lives.
To take a parable of Jesus and invent an allegory by assigning meaning and significance to each and every detail of the parable is risible, from my perspective, and poor hermeneutics to boot!
That there are, or may be, points of comparison between the details in the parables and other verses or themes in the Bible, does not vitiate the importance of both interpreting the parable in the context in which Jesus told it and extracting the moral from the parable and applying it to one's life.
For example, Jesus told the parable of the wise man and the foolish man who both built houses, the former on rock and the latter on sand. Was Jesus attempting to teach his audience that He was the rock, the chief cornerstone, a term with which Paul and Peter would later identify him (e.g., Ephesians 2:20 or 1 Peter 2:4-8)? No. Is there a common theme or thread running through the two appearances of the concept of a rock? Yes, but that common thread is composed of two things attached to each other in a posteriori fashion. Sometimes the attachments are good and sometimes not.
Occasionally a given concept, such as that of a rock, can appear earlier in Scripture, and a later writer picks up on it and applies it in a way the original writer did not. For example, God provided water for the Israelites in the wilderness when God told Moses to speak to the rock, which he did, and out gushed water. Paul in his letter to the Corinthians picked up on this concept of the rock and applied it explicitly to Christ:
"and all [the Israelites] drank the same spiritual drink, for they were drinking from a spiritual rock which followed them; and the rock was Christ" (1 Cor 10:4).
One could prepare a decent sermon by focusing on the different ways in which a rock functions as a symbol for a truth about God:
Christ is the spiritual rock who follows all His children, providing them with living water that truly pleases and satisfies throughout life
Christ is the rock-solid foundation, the cornerstone of the spiritual building that is His church universal
Christ is a rock of offense to those who choose to reject Him
Christ's words are like a rock-solid foundation that cannot be moved or shaken in the storms of life if we build our lives on them
God is a rock, a mighty fortress, a strong tower to those who take refuge in Him, as we learn from the book of Psalms.
Just as Moses was protected from seeing the effulgent and blindingly resplendent glory of God as God passed by him while Moses was shielded by the hand of God while Moses stood in the cleft of the rock, so too are Christians protected from being consumed by God by virtue of their being in Christ.
Christ could very well be the stone of which Daniel speaks, "a stone cut out without hands . . . [which] struck the statue on it s feet of iron and clay and crushed them. Then the iron, the clay, the bronze, the silver and the gold were crushed all at the same time and became like chaff . . .. But the stone that struck the statue became a great mountain and filled the whole earth . . .. [and] the God of heaven will set up a kingdom which will never be destroyed, and that kingdom will not be left for another people; it will crush and put an end to all these kingdoms, but it will itself endure forever" (Daniel 2:34,35,44,45).
In conclusion, the moral of each of Jesus' parables stands alone as a point of comparison between Jesus' audiences and their lives. To force the template of a full-blown allegory onto a parable is a risky business indeed. You might even say it's a slippery slope (notice the simile/analogy) that ends in confusion rather than enlightenment and God-inspired conviction.