Whether one considers Norse, Greek or any of the myriads of other mythologies around the world, one is often struck by their apparent similarities. When the cultures that gave rise to them are as closely related as the Norse and Greek ones, the similarities can be partly explained by their common Indo-European roots (as is also the case with the ancient polytheistic traditions of India, Rome, the Celts etc.), but when mythologies as far-flung as those of Australia, Meso-America and Japan show many of the same characteristics, the similarities are less easily explained:
Why is Mankind so often created from clay?
Why is the theme of the great flood so common as to be nearly universal?
Why do great skygods so often battle serpent- or dragonkind?
And what is the deal with all those dragons anyway?
Many explanations have been given to these similarities, both scientific and pseudoscientific ones, but as always, the simplest, with the least number of assumptions made, is likely to be the correct one (thanks, Occam 👍). Thus, we can rule out Ancient Aliens or Divine Inspiration and the like, since these explanations make grand assumptions concerning mighty exterior forces outside the human sphere, which have never been proven to exist and are undetectable, unmeasurable, untestable and therefore unfalsifiable as well as unscientific.
Rather, one should probably assume only one thing ... that we all share a common condition; the human one. This means that we have the same sets of approximate fears, needs and experiences wherever we happen to be born. We also share but one planet, on which we follow the same natural laws, share the same resources and are subject to the same climate, atmospheric phenomena and global natural disasters etc. Our remarkable minds; unique in their ability to construe believable stories to explain it all, and in their ability to make these stories into collective truths, have then given rise to myths which are all based on common themes, as these themes are what makes us us.