One effective method for beginning the introductory paragraph of an essay in the use of a quotation that is relative to one's thesis. With regard to the main characters of Shakespeare's romantic tragedy, a citation from another tragedy seems appropriate. It is the observation that Cassius makes to Brutus in Act I, Scene 2 of Julius Caesar as he tells Brutus,
"The fault, dear Brutus, is not in our stars,
But in ourselves, that we are underlings." (1.2.141-145)
Indeed, the results of many of the actions of the main characters are the fault of their personalities and characteristics, not that of fate as they may suppose. For, it is often their impulsive and designing actions as well as their attempts to escape fate which actually bring about their deaths.
Therefore, you may wish to prove that it is the character flaws of Romeo, Juliet, Tybalt, and Mercutio which bring about their tragic ends rather than happenings of Chance. And, by perusing the play again, you will find the support that you need. With Romeo, for instance, his becoming enamored of Juliet so suddenly that he scales the walls of the Capulet orchard and pledges his love to Juliet and rushes out to have Friar Laurence marry them without anyone's knowledge lays the path to their demise. Likewise, Juliet's failure to heed her own warnings that Romeo's pledge of love is "too rash, too unadvised, too sudden" (2.1) proves to be her undoing as well as she reacts in desperate ways when, having already married Romeo, she is betrothed by her father to Paris. Certainly, Tybalt's and Mercutio's choleric temperaments invite disaster, as well.