Why did Arwen do that??

[Mister Underhill]

Further to the question of Arwen’s ability (or lack thereof) to choose to stay in Middle-earth for years after Aragorn’s passing, here’s more from HoME XII. This is draft material from the Tale of Years, and while it is admittedly clear that much later revision of the story took place, this may be a case where an early sketch can provide some insight into the meaning and intentions of the final text:

King Elessar and Queen Arwen reigned long and in great blessedness; but at the last the weariness came upon the King, and then, while still in vigour of mind and body, he laid himself down after the manner of the ancient kings of Numenor, and died, in the hundred and second year of his reign and the hundred and ninetieth year of his life. (*These numbers obviously were altered later.)

Then Arwen departed and dwelt alone and widowed in the fading woods of Lothlorien; and it came to pass for her as Elrond foretold that she would not leave the world until she had lost all for which she made her choice. But at last she laid herself to rest on the hill of Cerin Amroth, and there was her green grave until the shape of the world was changed.

P.S. -- You really ought to hatch more of these devious schemes of yours, Squatter old boy.