from his recent ancestors.  Besides all of the fate stuff that we can take for granted...actually, it kinda does have to do with fate.  Aragorn is sort of the last link in the chain on many levels.  He is the one who has been preparing to restore the Kingship to Gondor(thus reconnecting the split of Isildur and Anarion's lines), he is the one who has the chance to unite with an elf(I didn't see Aragorn's grandpappy courting Elronds daughter), he is the one who will usher Men into the Fourth Age.  He has been preparing for all of these things like none of the other decendents of Elendil have done.  He knows and understands so many things that his forefathers did not.  Raised by Elrond, Counseled by Gandalf, traveled over larger portions of Middle Earth probably more than anyone.  Everything he has done in his life has prepared him for facing the most powerful forces in the land: the lure of the Ring, the Nazgul, facing Sauron in the Palantir, he even would have stood against the Balrog if not ordered by Gandalf.  As you say, he may be filled with innate doubt, but there is no person more capable to accomplish stuff than he was.  As we go on with our chapter discussions, we should never need to ask why Aragorn was capable of doing something, but, rather, why he screwed some stuff up despite his capabilities.

oh, and just a thought about the attack.  Why wasn't the Black Breath employed here?  It seems like a fairly awsome power, maybe the only one other than fear that the Nazgul have in their possession.