The play 'Romeo and Juliet' is both a love story and a tragedy. The essay can be written in two ways
Case 1. When the angle of 'love' is explored only
Case 2. When both 'love' and 'tragedy' are explored.
Topic Sentence for Case 1
Romeo and Juliet experience 'love at first sight' and Shakespeare's ‘Romeo and Juliet’ showcases how love transforms a young rational Juliet into a witty, loyal, emotionally intense lover and wife on one hand and a unusually impetuous young Romeo into a passionate committed lover and husband.
Then you can go on and incorporate that Romeo and Juliet is a story about the undeniable power of love.Romeo is infatuated with Rosaline, who does not reciprocate his feelings.Juliet is betrothed to Paris by her father.But destiny brings them together. You can elaborate on how their love for one another is different from their feelings towards Rosaline and Paris respectively. You can talk about their meetings and whether people around them notice the changes in their personalities by giving quotes from the text.
Case 2:The 'victory' of the bitter feud between the Capulets and Montages over the passionate love between Romeo and Juliet in the Shakespearean drama 'Romeo and Juliet' highlighting that so many tragedies can be avoided if we extend a little bit of love to one another.
You can proceed to expand Juliet's famous balcony scene, "It is too rash, too unadvis'd, too sudden.", their secret marriage, their love, Romeo’s banishment, the death of Mercution and Tybalt, and the ultimate suicide of the lovers.
Thus the choice of topic sentence will depend upon whether you will follow the thread of love alone or love and tragedy.