This thread is technically about "Lycanthropy and Skinwalkers" but it also includes discussion of dragons and also includes a good answer from u/itsallfolklore about the problem with treating any myth, but especially broad and complex sets of loosely similar myths as having a single root cause.

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Relevant specific quote:

Some people have speculated that there are inherent fears built into the shared human experience – including a fear of snakes – which has caused dragons to emerge as a worldwide motif, manifesting as a beast to be feared. That is pure speculation, completely unfounded on anything, and its flaw is demonstrated by the fact that many cultures have a beloved “dragon” tradition (so-called, again, by the convenience of a translated word). Some “dragons” are, in fact, kindly, lucky fixtures in folklore, bearing very little resemblance to the classic, feared, European dragon.