I would like to add the 'Can't change the way of his/her kind'
probably represents how Beren cannot dwell in the halls of Mandos as a Man, and neither can Luthien travel beyond the heavens where the souls of men wander, as an elf. And therefore it takes Luthien becoming Mortal and living a 2nd life with Beren for them to spend eternity together.
hmm.....Necro has the idea methinks
I would like to add the 'Can't change the way of his/her kind' probably represents how Beren cannot dwell in the halls of Mandos as a Man, and neither can Luthien travel beyond the heavens where the souls of men wander, as an elf. And therefore it takes Luthien becoming Mortal and living a 2nd life with Beren for them to spend eternity together.