Joseph Campbell identifies three characteristics of Myth that typify the effect they have on human beings and define their purpose.
The first is that they keep us from dying by reassuring us that all life comes from death and we must therefore feed on it to survive.
The second is that we must procreate, so that the species and, specifically, the tribe, can survive beyond us.
The third is to codify how we as a society should interact with other societies and their respective Myths.
The first two are obviously ways for the Myth itself to keep being told: both the individual and the society must persist in order to perpetuate the Myth in question. But the third is rather a reflection of the second on the Myth itself: the interaction of different societies is the main way Myths have to procreate.
Myths are alive, by the way, in case you hadn’t noticed.
But the weird thing about this interaction is that it doesn’t specify how the Myths will interact with each other. Quality #1, above, is basically “Kill or be killed”, quality #2 is essentially “Make up and make love,” but quality #3 could go any which way. How do Myths interact with each other?
Sometimes they’re polite and unobtrusive, sometimes they’re social and amenable, but sometimes we find there are Myths who are cruel and sollipsistic, Myths who insist that they—and only they—are Truth. They preserve their immortality by refusing to procreate, as though, like Zeus, they remember how they killed their own father and refuse to have the same done to them.