On the Making of Fire
Feanor grew swiftly, as if a secret fire were kindled within him. (Silmarillion 6: Of Feanor)
I shall never see my mother again, for so the Valar have decreed.
Indis is kind, and loves my father. Though I wish her gone I do not blame her, nor her golden-haired sons. She dances with my father at night, among the trees of Eldamar.
And my mother lies in the Halls of Mandos, a spirit without a body, sworn to never return.
They sat in judgement on her, when she fled her body in weakness. They offered mercy, the Valar's mercy: a wife for Finwe, a husband for Indis, a foster-mother for poor orphaned Feanor. But oaths are binding, and may not be revoked. My parents' marriage must stand until Arda be remade, or at least while life remains for my mother. So they offered mercy.
Mother was brave, they tell me, when she stood before Mandos. She pleaded for the sake of Indis, and her long-unrequited love. She pleaded for the sake of Father, and his loneliness and sorrow. She stood strong and proud among the Valar as she accepted the doom of Mandos:
If the Living is permitted to remarry, then by doom Mandos will not permit the dead to return.
And so the Law is preserved. And Mother is gone.
Father wants me to grow brave and proud and loving, like Mother, desiring the happiness of others. He tells me the Valar are kind, and will not refuse the request of a generous heart.
But that is not what I learn from the tale.
When I am grown I will vow only once, and hold to my vow. I will not let justice, or mercy, or kindness keep me from it. For the vow of the Eldar is as law, and is worth more to the Valar than my mother's life.
I will love only once, and, as the Valar have taught me, I will love only that which does not fade.
This fragment is based on the Statute of Finwe and Miriel in Morgoth's Ring. I do not consider this to be a canonical text, and I rather hope that Elven marriage did not actually work this way. Still, it goes a long way towards explaining why Feanor turned out the way he did.
Thanks to Finch for fact-checking, and to greenleaf-legolas for telling me to write this.