Love has always been a dagger in the back for Elrond.
For love, of sea and duty and family, Eärendil Ardamírë leaves his wife and twin sons behind forever. He manages to, eventually, accomplish his goal; he gains his wife, but he loses his children in the process.
One, he may meet thousands of years later. Another, not until Arda is remade, and maybe not even then.
The sword stained with the blood of his people is pulled from from Elrond's gut of Elrond's child-heart and offered hilt first. Gradually, Elrond will learn to grab for not the sword, but the hands which offered it, and he will love them, those permanently red-stained hands, against all thought and logic. Though he will offer that it was logical, too, to love them as well as hate them.
That is alright, he will think, because he has experience already with mingled love and hate.
The sword becomes a knife thrust into his back the moment the hands he has learned to love so very much push him away for his safety (for love of him) and then years later draws blood once more, of red (again) and not black.
Love is a dagger to the back, is a pronouncement of abandonment. Elrond has learned this lesson twice by the end of the War of Wrath, and the gong of fateful dread pounding in his heart when he and his brother are called to appear before the Herald of Manwë informs him he will have at least a third lesson to really let it sink in.
His brother, his twin, the only other on these shores that shares the unique blend of three kindreds, has chosen to cleave to a Fate opposite him. Elros may tarry awhile on this shore, and Elrond might even be able to visit his kingdom after it is established on the new, bright star-shaped island, but Elros will leave him behind. Utterly.
It is well, then, that the dagger which Elros so steadily wields is not shove into Elrond'd back, but into his heart, fast-forward. It does not make the wound hurt any less for all that he sees it coming, but there is an honesty to it which Elrond will, in time, learn to appreciate.
When Celebrían, his beautiful, silver-queen wife, is brought back to him scarred, marred, and broken, he nearly expects the leaving of her too. He fights against it, of course. He has ever fought against Fate, it seems.
But when she sails he can at least allow for a bit of hope. Hope that she will be well, in time. Hope that the knife which has serrated his heart so as to make it consciously bleed will eventually come out and he can heal as well.
Arwen's Choice is not so much a knife or a dagger but a sword and spear. It cuts him deeply, pierces him throughly. Where once, he might have been able to resist boarding a ship for a time despite the greater loss of Vilya's Power entangled with his Fëa, he cannot wait long at all.
That Elladan and Elrohír choose to remain behind—for a time, they say—merely deepens the fatal blows.
His children's Choices were meant to be made when he Sailed. He does not know, cannot hope, what it means that his twin sons have not explicitly stated their Choice and will not Sail with him.
The father in him is both thankful and mournful that they stay behind, in part, to watch over their younger sister. The wounded boy over six-thousdand years old counts it as yet another lesson, yet another blade which cuts deeply.
Love is a dagger to the back, a knife to the throat, a sword in the gut. Love is a blade that cuts and pierces and stabs.
Love is a blessing and a curse, and Elrond has known all his loves to be both.
