The method to see new phenomena as different occurences of older ones is part of the process of creating a myth. In my opinion it is also a reason for the existence of myths, because people use myths not only to express basic opinions of their society, but also to explain new phenomena and happenings within their society. To summarize this a bit I would say a myth is always something which comes from the past, which is used to interpret something in the present and which uses idealized images or concepts of the society to do so. This is surely an image which is not clearly shaped, but the following points should help to make it more precise. The roots of every myth are to be found in history.
Every myth has a more or less true historical background.
Most myths of course have very few things in common with the real history, but they developed out of it. By continuous usage and inheritance a myth expresses more and more the ideology of the society it is used by. The mythology of the ancient states of Greece for example represented the Greek form of government and ideology. Due to these three factors a myth becomes more and more a collection of symbols, icons and also stereotypes during time. Those symbols or icons inhabit a complex system of associations and it is possible to express such a system by using only one phrase.