The prologue of the plays introduces themes of fate and determinism, which is where the phrase “star-cross’d” originates from.
It gives the play a sort of nihilistic feel because we know from the onset that no matter what the lovers do, they cannot escape their fate. Shakespeare show fate to be insurmountable (cannot overcome), attributing a huge amount of power to the stars.
Furthermore astrology links to horoscopes. Juliet is born on Lammas Eve which makes her a Leo. She is an exemplary Leo, fiery and determined. This reinforces the power celestial bodies have over humans.