Aragorn probably could take the Kingship now if he wanted to do so.
Faramir has acknowledged him as King; so has Prince Imrahil. The Council
of Gondor, seeing that Gondor desperately needs a strong leader, would probably
recognise his claim if pressed.
All the same, it's important to Tolkien to present Aragorn
as not pressing his claim. More than that: Aragorn has to prove instead
that he can give up the Kingship when it is his for the taking. Instead
of claiming the Kingship he will opt to go on what he knows may well be a
suicide mission purely to buy Frodo more time. He earns the Kingship,
finally, by showing that he can let it go. That's the final test he has
to pass. This is Aragorn's equivalent of a nominated bishop's required
response in answer to the question if he agrees to be a bishop: "nolo
episcopari," "I do not wish to be a bishop." Only if he can say "no," is
he really worthy of the
office.
_______________________________________
The Nazgūl never did find out who it had been who -- during their unsuccessful attack on Strider and the Hobbits at Weathertop -- had applied glue to their horses' saddles. Certainly the Nazgūl, upon attempting to dismount after a hard night's ride, had not considered the matter amusing.
