Det enda svaret finns i samma brev du citerade. Här är en större del av brevet:
"For in his condition it was for him a sacrifice to perish on the Bridge in defence of his companions, less perhaps than a mortal Man or Hobbit, since he had a far greater inner power than they; but also more, since it was a humbling and abnegation of himself in confirmity to 'the Rules': for all he could know at that moment he was the only person who could direct the resistance to Sauron successfully, and all his mission was in vain. He was handing over to the Authority that ordained the Rules, and giving up personal hope of success.
...So Gandalf sacrificed himself, was accepted, and enhanced, and returned. 'Yes, that was the name. I was Gandalf.' Of course, he remains similar in personality and idiosyncrasy, but both his wisdom and power are much greater. When he speaks he commands attention; the old Gandalf could not have dealt so with Theoden, nor with Saruman. He is still under the obligation of concealing his power and teaching rather than forcing or dominating wills, but where the physical powers of the Enemy are too great for the good will of the opposers to be effective he can act in emergency as an 'angel' - no more violently than the release of St. Peter from prison....
Gandalf really 'died', and was changed: for that seems to me the only real cheating, to represent anything that can be called 'death' as making no difference... He was sent by a mere prudent plan of the angelic Valar or govenors; but Authority had taken up this plan and enlarged it, at the moment of its failure. 'Naked I was sent back- for a brief time, until my task is done'. Sent back by whom, and whence? Not by the 'gods' whose business is only with this embodied world and its time; for he passed 'out of thought and time'. Naked is alas! unclear. It was meant just literally, 'unclothed like a child' (not disincarnate), and so ready to receive the white robes of the highest. Galadriel's power is not divine, and his healing in Lorien is meant to be no more than physical healing and refreshment."
- The Letters of J.R.R. Tolkien, #156
Tjänsten (alltid kapitaliserad) som Tolkien talar kan vara ingen annan än Eru Ilúvatar. I det här fallet, som i alla andra, när Eru utövar sin makt, beror det på att det här är hur han har bestämt att saker ska vara. Eru skickade Gandalf tillbaka till Mellersta jorden eftersom han alltid hade för avsikt att göra det. Det var alltid hans plan. Gandalf är den som Eru valt att övervaka Saurons förstörelse, och Gandalfs beslut att offra sig för att rädda fellesskapet är den handling som gjorde honom värd för uppståndelse i Erus ögon.
Detta är en funktion av Tolkiens djupt hållna religiösa övertygelse. Eru är för honom väsentligen ett annat namn för den kristna guden. Det skulle nog inte vara meningsfullt för Tolkien att fråga varför gud gör vad han gör. Gandalfs uppståndelse var vad Tolkien kallade som "eukatastrop":
For it I coined the word 'eucatastrophe': the sudden happy turn in a story which pierces you with a joy that brings tears (which I argued it is the highest function of fairy-stories to produce). And I was there led to the view that it produces its peculiar effect because it is a sudden glimpse of Truth, your whole nature chained in material cause and effect, the chain of death, feels a sudden relief as if a major limb out of joint had suddenly snapped back. It perceives – if the story has literary 'truth' on the second plane (for which see the essay) – that this is indeed how things really do work in the Great World for which our nature is made. And I concluded by saying that the Resurrection was the greatest 'eucatastrophe' possible in the greatest Fairy Story – and produces that essential emotion: Christian joy which produces tears because it is qualitatively so like sorrow, because it comes from those places where Joy and Sorrow are at one, reconciled, as selfishness and altruism are lost in Love.
- ibid, #89