Depends if there is true simultaneous and isolated development of the same concepts or (more likely, with the level of trade that has gone on around the world for far longer than is commonly held) that there has been a greater deal of cross-pollination than people would at first expect.
After all isn’t our word ”dragon’ derived from the Chinese/Japanese word? Isn’t the more traditional European word ”Wyrm’?
On that note don’t Chinese/Japanese Dragons tend to be more celestial creatures rather than the Flesh, bone and Winged monstrosities of European folk tales?
On the whole, a lot of the supposed commonalties simply aren’t there. They arise mainly through making glancing comparisons from surface impressions and not the actual understanding of the myths and messages. It’s like confusing Mayonnaise and Salad-Cream: at a glance they seem the same but are very different.
The point I’m making is, after all the many hundreds of years that man has been trading with the nations of the world people would have told stories, and people from other places would say hey that’s a little like our story of . .. and so there is opportunity for both versions become a little altered from their encounter. Over the years the fact that this cross-pollination happened gets forgotten and people marvel at how similar the stories really are.
Anyway, pushing the possible contamination of the original stories aside, we would also have to bring “Steam Engine Time” to the table. (see Science of the Discworld III) this is the notion that sometimes it is simply an Idea/concept’s time to be. When this happens then lot’s of people will be having similar thoughts and idea’s at the same time.. You see this in the movies and games and those kind of outlets for human creativity. Suddenly out of nowhere people with think Hey what we need is a good movie set on the ocean in period boats. So you get Pirates of the Caribbean and Master and Commander. I’m sure there were others out there in other countries and different media but you get the idea.
I’m in the Games industry and the number of times I’ve thought of a great idea for a game and then discover the next week that someone else has begun work on a very similar idea. So if it happens that frequently in such a narrow field then it should be no surprise that the same flavours and themes crop up in disparate ancient mythology. It’s simply how the human mind is wired. It creates similar solutions to similar problems no matter where you are or who you are.
But finally as with most comparisons we only notice the close similarities and go that’s odd. We fail to register the thousands of stories that don’t match up.
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