Aragorn é o herdeiro de Isildur
Sendo o legítimo Rei dos tronos de Gondor 1 (e Arnor restabelecido), ele teve o potencial de reunir os reinos dilacerados dos Dúnedain: Arnor e Gondor, para se opor a Sauron.
Then Aragorn, being now the Heir of Isildur, was taken with his mother to dwell in the house of Elrond; and Elrond took the place of his father and came to love him as a son of his own. But he was called Estel, that is "Hope", and his true name and lineage were kept secret at the bidding of Elrond; for the Wise then knew that the Enemy was seeking to discover the Heir of Isildur, if any remained upon earth.
A maior razão pela qual Sauron ficou com tanto medo foi:
Sauron (erroneamente) acreditava que Aragorn tinha o Anel e usaria contra ele
'Now Sauron knows all this, and he knows that this precious thing which he lost has been found again; but he does not yet know where it is, or so we hope. And therefore he is now in great doubt. For if we have found this thing, there are some among us with strength enough to wield it. That too he knows. For do I not guess rightly, Aragorn, that you have shown yourself to him in the Stone of Orthanc?'
'I did so ere I rode from the Hornburg,' answered Aragorn. 'I deemed that the time was ripe, and that the Stone had come to me for just such a purpose. It was then ten days since the Ring-bearer went east from Rauros, and the Eye of Sauron, I thought, should be drawn out from his own land. Too seldom has he been challenged since he returned to his Tower. Though if I had foreseen how swift would be his onset in answer, maybe I should not have dared to show myself. Bare time was given me to come to your aid.'
Portanto, Sauron tinha mais motivos para temer o poder de Aragorn. Apesar de ser um homem mortal, Aragorn 'arrancou' a pedra Orthanc contra a
vontade de Sauron . Isso definitivamente é ter medo de Aragorn.
Eu não acho que Aragorn tendo uma quantidade muito pequena de sangue Maiar nele era a razão pela qual Sauron o temia, como muitas das outras respostas dadas parecem sugerir. Na época em que Aragorn nasceu, seu sangue Maiar estava tão diluído que pode ser considerado irrelevante. Em
esta excelente resposta a porcentagem de sangue de Maiar de Aragorn é calculado como
0.000000000000000001355%
.
1 : Isso é perfeitamente respondido aqui .