How does Juliet's refusal to bow to the pressures exerted by her parents and the Nurse reinforce the idea that tragedy is the only outcome that can be expected from these circumstances?

Because there is such a strict societal "norm," we know that Juliet's rebellion was viewed as headstrong and willful, and neither of these is desirable in a lady. When others turn from her, she will become desperate because she is so used to being catered to and may act rashly