He had lied to Dionysius by telling him that the reason for his prohibition was that he, Zeus, was interested in Sigda, as if there were not a thousand other more nymphs worth his time. The true reason, though, was a prediciton of Apollo, that said that if Dionysius allowed himself to fall for the nymph, in the end it would all come to a decision, and someone, a god, would not choose well.