Juliet's tragic flaw is her loyalty to Romeo. She loves him and is so loyal to him that she could not bear to live without him. So when he died, she had to die as well--to be forever with him. She was willing to take the potion that made her appear dead--even though it scared her that it might kill her all because she would not be forced to marry Paris. Her loyalty was her strength, but it was also led to her demise.
The play begins with the Prologue--and that is where it tells us of their fate. "Two star-crossed lovers take their life." That theme is present throughout all of Shakespeare's works. Any mentioning of the stars refers to fate and what has already been decided. It's as if the characters have no control over their lives or over their decisions and actions. They must accept their fate because they can do nothing to alter it.
juliet is capulet ,dutiful child ,docile.she is shy,loyal from the begining of the play.she shows her obedience to her parents.on the other hand she falls in love with romeo and she vows love for him.her tragic flaw is the fluctuation between her love to her family on hand ,on the other hand the love to romeo.she should have decided from the begining of the play either marrying romeo or follow her family by marrying her cousin paris.instead of restoring the love between the two families ,they widened the gap since romeo murdered her cousin paris due to the murder of his friend mercutio .her tragic flaw leads to the their tragic death at the end of the play.
the role of imagination has been mentioned at the begining of the play.since romeo a dreamer it could be that he sees his fate by imagination.so they commit suicide by fulfuling the willing of the god.