"Dragon" Stories are not true stories of dragons; rather, they are allegorical tales about something that should not be talked about openly. Some believe that the Book of Revelations in the Bible is the best example of this; however, with the various interpretations of the Bible, it is best to review another story.
The Dragon of Wantley is perhaps the best myth to review for this purpose. It can be found in Percy's Reliques, which recounts the tale.  The tale says that a knight, More of Mere's Hall (in other sources known as More of More Hall ) fights a mighty dragon which was, like in many myths, the scourge of the country side. He wore a special suit of armor, which was covered in spikes, and he then strode off to fight the mighty beast. The fight itself is depicted somewhat humorously, for he kicks the dragon in the rump, and the dragon impales itself upon his spiked armor. 
What this tale real conveys, although it is up to the individual to interpret, is actually a court battle which fought feudal oppression. The Dragon of Wantley ate animals and ruined lands, which can be metaphor for a lord stealing most of the peasants' money.  (Most point to Nicholas Wortley and his successor as the lords. ) The fact that the actual fight is depicted almost mockingly has led many to believe that this story had absolutely nothing to do with a dragon, but with a person so horrible that he deserved the name "dragon". 
Obviously, however, most dragon myths were not derived this way until later, when the dragon had already been given its name and reputation as a destroyer of all things. However, these allegorical stories should not be simply passed by, for they tell a tale that would otherwise been forgotten because it would have been forbidden to tell.
The Connection to the Mother Goddess
Mother Goddess was a very large part of early Earth worship, and she was commonly associated with serpents.  The worship of the Mother Goddess was derived from the awe of birth and menstruation, which men could not understand fully at the time.  Most religions, therefore, centered around not just earth worship, but also the worship of women and the Goddess.
The complexities of the Mother Goddess worship are not fully known today, although there is some evidence to show that it began to lose favor with the people. Some will blame Christianity for bringing down this worship, but those people are ignoring other important reasons for the fall of the worship of the Goddess.
Perhaps the biggest factor, although it is not certain, is the fact that childbirth was very dangerous. Many women died from it, for people did not have a good understanding of health at that point.  Another reason is that some of the worship around the Goddess was bloody and some even required sacrifice. For this, and many other unknown reasons, the Mother Goddess started to fall out of favor with people.
What does this have to do with dragons? Serpents were commonly connected to the Mother Goddess; however, the reason for this connection is still quite unclear. It can be seen around the world in such examples as a Gorgon. Notice that the common dragon slayer myths often include a woman to be saved by the valiant knight. 
The theory today states that the societies with a more patriarchal worship came down through the Mediterranean area, where many of the Mother Goddess cults remained, and changed the culture through war and conquest. This can account for the many dragon slaying myths that have been recorded.  The most common format for the story is something like this: An evil dragon scourges the land, a valiant knight takes up the charge of killing the dragon, and he slays the dragon and wins a bride or saves the virgin sacrifice. While some interpretations may vary, the knight is generally a symbol of all that is good, while the dragon is a symbol of corruption. The knight, which represents the new conquering culture, should be revered by the people while the dragon, which represents the Mother Goddess of the past, should be hated. What is left of the symbolism of women? Well, she's either give over as a prize, or she is a virgin saved by the knight. 
Even before Christianity, the Mother Goddess was devalued. However, she was not tossed aside entirely. There was still a need for a mother figure in religion; her complete "overthrow" did not fully take place until the Christians took their toll upon the religions.  In the end, the people were left with two major role models for women: the Virgin Mary and Eve. 
Connections between women and serpents and dragons are everywhere. Tiamat was slain by Marduk, a male god. Eve was tempted by a snake; the Gorgons had hair that was writhing with snakes. Apollo slew Python, a serpent of terror. The saints commonly slew dragons to save maidens. Countless connections can be made between dragons and the view of women. 
Despite the fact that the Mother Goddess connection can account for many of the dragon myths, it cannot account for Eastern dragons at all. (Ironically, the dragon was the symbol of the Emperor while the Phoenix represented the Empress.) And, furthermore, it does not explain all dragon myths fully.
The Dragon as a Symbol
The beautiful thing about symbols is that they are adaptable even to today. Much like in the past, where people were sometimes forced to used allegory to retell a story, symbols can be used to describe people or things. For instance, many groups adopted the symbol of a patron saint slaying a dragon. Examples of this include The Black Hundreds of Russia and the Black Dragon Society of Japan.  These groups adopted the dragon as a symbol in both a negative and positive light.
By using the dragon in modern history as a symbol, it perpetuates the likelihood of more dragon myths being created. Many political books now include the word "dragon" in their titles, usually referring to the person or the area that the book focuses on.
For more information on footnotes and references, please see the bibliography.