A few different themes you could explore in Romeo & Juliet are:
Love is the dominating theme in Romeo & Juliet. More specifically, examine the way love controls these two young people and causes them to act hastily and without consideration for consequences. They love each other, but they lack the maturity to deal with that love in a constructive way. Their love for each other causes them to abandon their family and friends. Romeo abandons his friends Mercutio and Benvolio after the party in order to seek out Juliet. Juliet tells Romeo to "deny thy father and refuse thy name...or wilt thou not, but be sworn my love, and I'll no longer be a Capulet." Love also leads to defying the rules that govern them. Romeo, banished from Vernoa, ignores the Prince's ruling and sneaks back in to see Juliet anyway. This ultimately leads to both their deaths.
2. Love as a cause of violence
Shakespeare explores the chaotic side of love in this play. Although a love story, it is full of violence and death. Their relationship is marked by death from the moment it begins. At the feast where they meet, Tybalt claims he will kill Romeo for his intrusion. Tybalt does end up killing Mercutio over Romeo's actions, and Romeo in turn kills Tybalt. Both Romeo & Juliet are haunted by suicidal thoughts over the course of the play. Their love cannot immediately supercede the hate that divides their families, so thoughts of dying rather than living apart control their thoughts and actions. Love and passion also lead to Paris being slain by Romeo at Juliet's tomb.
The theme of fate is a major part of Romeo & Juliet. Although their actions lacked criticial thinking and their own immaturity and failure to control their emotions led to their deaths, the idea this being their fate is the basis of the play. The Chorus calls them star-crosses lovers at the onset. Perhaps it was their destiny to die young. In addition to the audience being aware of this fate, Romeo & Juliet appear to be cognitive of it as well. Just before entering the Capulet house for the feast, Romeo has a premonition of something bad happening that will lead to untimely death. Later, when he believes Juliet is dead, he cries "then I defy you, stars." Their love seems to be in oppostion to what fate has in store. Examples of this include the extreme hatred between the families (which is never explained) that keeps Romeo & Juliet apart, the series of events that unravel Friar Laurence's plan, and the awful timing of Romeo's suicide just as Juliet wakes up. It seemed as if their fate was not to be with each other, but to die trying.
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