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[Puddleglum]

Originally Posted by Hookbill the Goomba

It is perhaps strange that the act of killing a dragon is seemingly reserved for people with human blood. Any thoughts on why?

I think it's been observed elsewhere that Tolkien seems to have set up Dragons as a primary mythical foil for mortals and Balrogs (or other demonic, as in "incarnated spirits" beings) as the parallel mythical foils for non-mortals.

So, just as we don't hear about Elves (at least not full elves) taking on Dragons, we likewise don't hear about Men taking on Balrogs.

The "why?" is less clear, but I suspect it is, at least partly, as a literary device - flavoring the two kindreds with another difference while still providing each with similar feats of heroism and accomplishment.