Several versions of Cyrus birth and rise to power are recorded. Herodotus (i.95) mentions three. In that which he quotes (i.107), it is said that Mandane was the daughter of the Median king Astyages, who, in consequence of a dream which he had had, foretelling the ultimate triumph of her son over his dynasty, gave her in marriage to a Persian named Cambyses, who was not one of his peers.
Taken from (
http://www.searchgodsword.org/enc/is...i?number=T2485)
Seems like a good reason.