First this humility reminds me of Christianity. Like the Christ, one should be humble (even if I'm not sure He was that humble all the time...). I have no idea how it is in King James or any other English translation but freely translated the Book says "The ones who uplift themselves will be downgraded - and the ones who downgrade themselves will be uplifted" (in Finnish translation they use the same words on both sides of the metaphor).
Secondly it might as well be linked to Tolkien's love of Kalevala and the Finnish folklore - and why not British modesty which is not too much unlike Finnish modesty. But talking about national myths, the Finns really think they are the most modest people in the world. (And how modest is that?)
*should we have a British-Finnish contest on which one is more modest?*
Surely Egill or Gunnar are not the most modest characters you can find, and neither is Wieland / Woland / Wayland / Völundur the smith or any of those ancient northern legends. Even Thor and Odin seem to be quite self-asserting - like King Arthur or Lancelot; or Roland or Siegfried to that matter. They are clearly pagan heroes even if the writers of their stories have instilled Christian features into their stories. Anyway, they have pride in what they do.
But the Finnish folklore is just filled with these ideas of being low key and not making a number of oneself (with the exception of Kullervo eg. Turín, and possibly
Ilmarinen eg. the smith eg. the S
ilmaril-maker) even before the christendom arrived here.
So maybe Tolkien was looking at that one genuine principle that would bring together the Christ-myth (Jewish) with the ancient stories (Finnish) that fit his view of the genuine idea of a good (British) common folk? And the utmost bravery of those we in Finland call the "silent of the earth"...