Love
Though Romeo and Juliet is arguably the most archetypal love story in the English language, it portrays only a very specific type of love: young, irrational, passionate love. In the play, Shakespeare ultimately suggests that the kind of love that Romeo and Juliet feel leads lovers to enact a selfish isolation from the world around them. Romeo and Juliet eschew their commitments to anyone else, choosing to act selflessly only towards one another. Sexuality does pervade the play, both through bawdy jokes and in the way that Romeo and Juliet anticipate consummating their marriage, but it does not define their love. Instead, their youthful lust is one of many reasons why their relationship grows so intense so quickly. Throughout the play, Shakespeare only describes Romeo and Juliet's love as a short-term burst of youthful passion. In most of his work, Shakespeare was more interested in exploring the sparks of infatuation than long-term commitment. Considering that no other relationships in the play are as pure as that between Romeo and Juliet, though, it is easy to see that Shakespeare respects the power of such a youthful, passionate love but also laments the transience of it.
Death
In Romeo and Juliet, death is everywhere. Even before the play shifts in tone after Mercutio's death, Shakespeare makes several references to death being Juliet's bridegroom. The threat of violence that pervades the first acts manifests itself in the latter half of the play, when key characters die and the titular lovers approach their terrible end. There are several ways in which the characters in Romeo and Juliet consider death. Romeo attempts suicide in Act III as an act of cowardice, but when he seeks out the Apothecary in Act V, it is a sign of strength and solidarity. The Chorus establishes the story's tragic end at the beginning of the play, which colors the audience's experience from the start - we know that this youthful, innocent love will end in tragedy. The structure of the play as a tragedy from the beginning makes Romeo and Juliet's love even more heartbreaking because the audience is aware of their impending deaths. The journey of the play is the cycle from love to death - and that is what makes Romeo and Julie so lasting and powerful.
Age
Throughout Romeo and Juliet, Shakespeare establishes the ideological divide that often separates youths from adults. The characters in the play can all be categorized as either young, passionate characters or older, more functional characters. The youthful characters are almost exclusively defined by their energy and impulsiveness – like Romeo, Juliet, Mercutio, and Tybalt. Meanwhile, the older characters all view the world in terms of politics and expediency. The Capulet and Montague patriarchs are certainly feisty competitors, but think in terms of victory as a concept, ignoring the potential emotional toll of their feud. Friar Laurence, who ostensibly represents Romeo and Juliet's interests, sees their union in terms of its political outcome, while the young lovers are only concerned with satisfying their rapidly beating hearts. While Shakespeare does not posit a moral to the divide between young and old, it appears throughout the play, suggesting that the cynicism that comes with age is one of the many reasons that humans inevitably breed strife amongst themselves. It also implicitly provides a reason for young lovers to seek to separate themselves from an 'adult' world of political violence and bartering.
Identity
Romeo and Juliet suggests that individuals are often hamstrung by the identities forced upon them from outside. Most notably, this theme is manifest in Juliet's balcony soliloquy, in which she asks, "Oh Romeo, Romeo, wherefore art thou Romeo?" (2.1.75). The central obstacle of the play is that the two passionate lovers are separated by a feud based on their family names. The fact that their love has little to do with their given identities means nothing to the world around them, and so they must choose to eschew those identities while they are together. Unfortunately, this act of rejection also means Romeo and Juliet must ignore the world outside their comfortable cocoon, and, as a result, the violent forces ultimately crash down upon them. A strong sense of identity can certainly be a boon in life, but in this play, it only forces separation between the characters. Even Mercutio, who is not actually a Montague, is killed for his association with that family. The liveliest characters in Romeo and Juliet die not because of who they are, but because of the labels that the outside world has foisted upon them.
Gender
In Romeo and Juliet, Shakespeare upends certain gender expectations while simultaneously reminding his audience that these defined roles do exist. Romeo arguably displays feminine characteristics, at least as defined by his peers. He ignores all calls to action, and has little use for the aggression that most males around him exhibit. His pensive nature is cause for his friends' mockery. Even after he falls in love, Romeo is far less prone to action than Juliet, who in fact shows a tendency towards efficient maneuvering that is otherwise exhibited by male characters in the play. She makes quick decisions, like her idea that she and Romeo should wed, and is not easily discouraged by bad news. In these two protagonists, Shakespeare is certainly reversing what his Elizabethan audience would have expected, as he frequently did with his heroines. However, the pressures on Juliet to get married – especially from Lord Capulet, who is interested only in a good match and uninterested in love – remind the audience that such atypical strength in a woman can be threatening to a patriarchal society. Juliet's individualism is quickly quashed by her father's insistence on a marriage to Paris, and though she ultimately outwits him, his demands are a reminder that the world of Romeo and Juliet did not value reversals of gender roles as much as the audience might have.
Revenge
Romeo and Juliet suggests that the desire for revenge is both a natural and a devastating human quality. From the moment that the play spirals towards disaster in Act III, most of the terrible events are initiated by revenge. Tybalt seeks out Romeo and kills Mercutio from a half-cooked desire for revenge over Romeo's attendance at the masquerade ball, and Romeo kills Tybalt to avenge Mercutio. Romeo's desire for revenge is so overpowering that he does not pause to think about how his attack on Tybalt will compromise his recent marriage to Juliet. Of course, the basic set-up of the play is contingent on a long-standing feud between the Montagues and Capulets, the cause of which no longer matters. All that matters is that these families have continued to avenge forgotten slights for generations. Though Shakespare rarely, if ever, moralizes, Romeo and Juliet certainly presents revenge as a senseless action that always causes more harm than good.
Marriage
In Romeo and Juliet, Shakespeare does not paint an attractive picture of the institution of marriage. The only positive portrayal of matrimony – between the titular lovers – can only be conducted in secret, and even Friar Laurence slightly disapproves because Romeo and Juliet have decided to wed so quickly. Shakespeare seems to be suggesting that marriage based on pure love does not belong in a world that abuses the sacred union. The manner in which Lord Capulet insists upon Juliet's marriage to Paris suggests both the way he views his daughter as object and the way in which marriage can serve as a weapon against a rebellious young woman. Even the religious figure, Friar Laurence, sees marriage as political; he marries Romeo and Juliet to gain the political power end the feud between their families, and not because he necessarily approves of their love. Ultimately, the central marriage in Romeo and Juliet ends in death, showing that this kind of passionate, irrational union cannot exist in a world fueled by hate and revenge.