...In grief, Lúthien lay down and died as well, going to the Halls of Mandos, where the spirit of the dead await re-embodiment in Valinor (for Elves) and departure from the circles of the world entirely (for Men).
There she sang a song of woe before the throne of Mandos Lord of the Dead, of the tribulations and suffering of both Elves and Men, the greatest ever sung, so touching that Mandos was moved to pity for the only time. As a result he summoned Beren from the houses of the dead and Lúthien's spirit met his once again by the shores of the sea.
Luthien knew that this would be their final meeting, since Beren could not remain on the earth beyond his time and she was thus faced with the prospect of eternal widowhood. Mandos consulted with Manwë, King of Arda. Even Manwë could not change the fate of Men, and so he presented Lúthien with the only choice possible:
To live in the immortal land of Valinor, where she could forget all her grief and enjoy eternal happiness along with her people and the Gods (Valar) but without Beren, or to return to the land of Middle-earth together with Beren as a mortal herself, accepting the Doom of Men and sharing in whatever unknown fate awaits them outside the Circles of the World.
She chose this latter option. With this she accepted death as a mortal woman, and although it wasn't the fate of her race, she turned creation upside down for her love of Beren and relinquished everything for him and became a mortal woman.