tolkiens legendarium - Why did Arwen have to become mortal to marry Aragorn if they were both half-elven?

I am sure Elrond is half-elven, so Arwen is too. I recall something saying Argorn was half-elven as well. Shouldn't half-elves not have to become mortal to marry each other?

Answer

Saying that someone is "half-elf" is a way of saying that they have an elven parent and a mortal one. Elrond fits into this category (his mother, Elwing, was an elf; his father Earendil was a human). But one is not thereby "half-human, half-elf" - one has to choose between being an elf and being a human:

"... Elrond Half-elven, who chose, as was granted to him, to be numbered among the Eldar; but Elros his brother chose to abide with Men." (Silmarillion chapter 34; page 254 in my edition)

Thus, at the time of the War of the Ring, Elrond is quite definitely an elf. Arwen gets that same choice.

But Aragorn is quite definitely a human; he's a descendant of Elros. Thus, he is going to die; and Arwen makes the choice to die as well, that is, to be under the Doom of Men, in order to be with him. It is only at the very end of her life that she sees what's involved with this and regrets it:

"If this is indeed, as the Eldar say, the gift of the One to Men, it is bitter to receive." (Lord of the Rings, Appendix A, p. 1038 in my edition)

She doesn't seem to ever reconcile herself to it.