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Well Sauron has a great deal of hatred for Luthien's descedants as a whole. This is further increased by the Numenoreans stopping him in the Second Age. More fuel is added to the fire when the Numenoreans give him a humiliating defeat under Ar-pharazon. Then Elendil makes the last alliance and plays a big part in personally killing his body. Lastly to add insult to the wound Isildur takes his ring. So purely from a personal point he would be angry to find out that a descendant of Isildur still lived.
The other thing that would greatly worry him is the Heir of Isildur claiming the One Ring. Technically Aragorn has a legal right to the ring, which even Frodo acknowledges at the Council of Elrond. Whilst Aragorn has no possible hope of ever defeating Sauron in a personal battle, there is a chance he could unite the armies of the West against Sauron. He may even be able to gain the Black Numenoreans to his side. With such military power, just like the Numenoreans did, Aragorn could defeat Sauron's armies.
The last reason is that it is still a huge boost for the men of Gondor and strikes fear into the orcs. As Boromir put it the 'sword of Elendil would be a help beyond hope.' Similarly I think it would be a huge blow to the elves to see Narsil reforged.
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