After Miriel died, Manwë decreed that Finwë could choose another spouse and he did get remarried to Indis, mother of Fingolfin and Finarfin; Fëanor wasn't too happy with that.
Why would Manwë allow Finwë to remarry if Elves only get married once?
After Miriel died, Manwë decreed that Finwë could choose another spouse and he did get remarried to Indis, mother of Fingolfin and Finarfin; Fëanor wasn't too happy with that.
Why would Manwë allow Finwë to remarry if Elves only get married once?
Before I get going, I want to correct a misconception: Manwë did not "allow" Finwë to re-marry, in the sense that he did not grant special dispensation. What happened was that Finwë was the first Elf to want (or need) to re-marry, so nobody knew if that was okay yet. What Manwë (and the rest of the Valar, in council) did was to fill the gap in the laws of the Eldar; after Finwë, any elf was allowed to re-marry (under certain conditions). It's just that not many did, because (from their perspective), this one ended so disastrously.
It's also a bit of a misconception that Elves only marry once; that's not actually an explicit part of their laws, but rather a natural consequence of two factors:
If you mate for life, and your life is unending, logically you will only marry at most once. That's all fine and dandy, but ignores the complication that it is, in a sense, possible for Elves to "die". This is further complicated by the fact that, in Aman, death is literally just another country; the Valar can go and ask Miriel what she wants, because she's right there, in Mandos.
There are sort of two parts to this, because the Statute of Finwë and Miriel was introduced in two stages:
The reason Mandos made it was because of Justice. Finwë and Miriel were in a bit of an odd situation, for three reasons:
In Mandos' educated opinion, there was no good reason to forbid re-marriage in this situation; despite the fact that death is a temporary inconvenience for Elves, Miriel's choice to stay dead made it a permanent condition. With no chance of a happy reunion with his first wife, why shouldn't Finwë be allowed to take another?
The debate of the Valar is the subject of a very, very long essay in Morgoth's Ring. It's far too long to include here in its entirety, but I'll try to pick out the highlights.
The gist of the debate is whether Miriel's death and Finwë's desire to re-marry is a consequence of Melkor's corruptive influence ("Arda Marred"), or if it's part of Eru's "True" conception of the world. If the former, then this is against Eru/God's law and should be forbidden; if the latter, it's the will of Eru.
While there are a lot of arguments back and forth over this question, Manwë's ultimate justification is essentially "Eru moves in mysterious ways." There's also an element of practicality to his decision; although the Elves are by nature immortal, death is nonetheless a reality of the world, and the laws of the Eldar need to take it into account:
The Valar have not and must not presume certainty with regard to the wills of the Children. Nor, even were they certain in this one case concerning the fea of Miriel, would that unmake the union of love that once was between her and her spouse, or render void the judgement that constancy to it would in Finwë be a better and fairer course, more in accord with Arda Unmarred, or with the will of Eru in permitting this thing to befall him. The Statute openeth the liberty of a lower road, and accepting death, countenanceth death, and cannot heal it.
History of Middle-earth X Morgoth's Ring Part 2: The Second Phase Chapter 3: "Laws and Customs of the Eldar" Of the Severance of Marriage
There are a number of reasons put forth for the Valar as to why they should allow this, but they essentially boil down to these two:
So because of those three reasons, Manwë and Mandos decided to allow Elvish remarriage, under certain conditions. Those conditions are outlined in the full Statute, also given in the text:
Marriage is for life, and cannot, therefore, be ended, save by the interruption of death without return. While there is hope or purpose of return it is not ended, and the Living cannot therefore marry again. If the Living is permitted to marry again, then by doom Mandos will not permit the Dead to return.
History of Middle-earth X Morgoth's Ring Part 2: The Second Phase Chapter 3: "Laws and Customs of the Eldar" Of the Severance of Marriage
There is one final reason, which Mandos hints at near the end of the debate; Mandos has a certain power of foretelling, and he predicts that allowing Finwë and Indis to have children will have massive (beneficial) consequences:
'Hearken now, O Valar! To me foretelling is granted no less than doom, and I will proclaim now to you things both near and far. [...] [T]he children of Indis shall also be great, and the Tale of Arda more glorious because of their coming. And from them shall spring things so fair that no tears shall dim their beauty; in whose being the Valar, and the Kindreds both of Elves and of Men that are to come shall all have part, and in whose deeds they shall rejoice. So that, long hence when all that here is, and seemeth yet fair and impregnable, shall nonetheless have faded and passed away, the Light of Aman shall not wholly cease among the free peoples of Arda until the End.
History of Middle-earth X Morgoth's Ring Part 2: The Second Phase Chapter 3: "Laws and Customs of the Eldar" Of the Severance of Marriage
And its worth pointing out just who is descended from this union: