1) fate cannot be controlled by humans / "star-crossed lovers take their lives," - were destined to die / Fate is omnipotent when Juliet says, "O happy dagger" - oxymoron shows she has no control / Many references of death and curses throughout play - "a plague on both your houses" (reference to plague that stops letter getting to Romeo.)
2) fate is a scapegoat for the character's own mistakes, eg Friar and Romeo / Romeo is a reckless character "fire-eyed fury be my conduct now" yet he blames his own actions on fate, "I am fortune's fool." he resents its control over his life, "I defy your stars."
3) characters who try to overcome fate always fail / Juliet tries to deny fate, but in the end it is inevitable. Despite this, she often foreshadows the future, "my grave is like to be my wedding bed". / Friar Lawrence is also aware of fate's power "these violent delights have violent ends" yet tries to deny it, ultimately being the cause of the pair's deaths who he was trying to prevent.