Why would Shakespeare have Juliet allude to Jove when she is talking to Romeo?

Shakespeare and most people with even a rudimentary education in his day had some idea of the Roman way of life and of their Gods. Referring to Jove would have been quite comprehensible to the audience.

Shakespeare could have his characters appeal to the Roman gods without fear of censorship, since officially they were not real gods, and things you said about them could not be blasphemy. It would be another thing altogether if you said it about God or Jesus. Juliet's line is "At lover's perjuries, they say, Jove laughs."; it would have been a whole different ball of wax had she said "Jesus doesn't care if lovers tell lies."