Chances are you've heard about the similarities between the Sumerian Epic of Gilgamesh and the Genesis flood account, or how Tolkien's Middle Earth was inspired by Scandinavian and Germanic folklore, or how the Romans stole the Greek gods and gave them different names. What accounts for the continuity among myths? Why is there always a creation story passed down in every culture? Who cares about "the gods," anyway?
Humans have a common story. It wasn't as if the Arapahoe Indians came into being independently of the Mandarin Chinese or the Celts; their mythologies might suggest otherwise at first glance, but on closer inspection similarities emerge. One of the most striking is the prevalent concept of a god or gods emerging from a void to create the universe. Another, particularly common in the Near East, is the story of one good man who survives a universal flood in order to preserve human and animal life.
Similarities persist beyond creation myths. Basically every mythology around the world includes a trickster figure—usually a deity, always a supernatural being, the trickster exists to tempt humans and other gods, cause problems (both benign and disastrous), and betray anyone who gets too close. In Norse mythology the trickster is Loki; for the Native Americans it was Coyote; in West African tradition he takes the form of a spider, Anansi.
There are differences, of course. Myths encapsulate a culture's religious, moral, philosophical, psychological, and scientific attitudes, and as far as any two cultures are different, their myths will reflect that. Eastern mythology sees things primarily as a unity, whereas the Greeks liked to compartmentalize (anticipating even in ancient times the Enlightenment dualism their philosophers grandfathered).
Yet even here, specific gods in both the Eastern and Western traditions were attributed certain tasks and functions. Gods frequently took other shape and appeared to humans, gods were essentially humans on a large scale, the mysteries of nature were explained by the activities of the gods (usually with a bizarre earthiness). All this is true in nearly every culture that has produced its own mythology.
Does that mean the universality of mythological types and archetypes proves the general principles to be true? Of course not—ancient myths are at odds with both Christian doctrine and modern science, and people that still believe them either live far from civilization or spend their Saturdays browsing New Age bookstore shelves. What it does mean is that human nature is pretty universal, and man's imagination is ultimately limited by the boundaries of that nature.
What myths show us best is not man as he actually is, however; they give us a clear picture of man as he would like to be. By creating stories of gods that are essentially humans without boundaries, man is able to envision his deepest fantasy, to himself be godlike. Yet it isn't the biblical holy and transcendent god man wants to be, it's simply a being actually capable of doing whatever he or she pleases without fear of consequences. The gods of myth are simply men whose rebellion has been granted license.
Reading the ancient accounts is about as instructive as you can get if you want to know how we humans think of ourselves. If the picture is less than flattering, that may be due to fits of honesty on the mythologist's part, or to the fact that as Christians we view nobility much differently. Reading the old myths is sometimes entertaining, often jarring, but always helpful if you're looking to understand Man as he really is.