Loyalty
The film centers around two rival gangs, the Sharks and the Jets. The main force keeping the members of each gang together is a sense of loyalty. Even though Tony has moved on from his life as a Jet and gotten a job, he still maintains a loyalty to his old friend, Riff, the leader of the Jets—"womb to tomb, birth to earth" is what they say to one another to show their loyalty—and he even kills his girlfriend's brother to retaliate for Bernardo murdering Riff. Even though it causes even more conflict, the characters in the film are loyal to one another based on gang affiliation.
Loyalty also comes up in relation to race. The reason that people object to Tony and Maria's love affair is that they believe that the two lovers should be loyal to their respective races. "Stick to your own kind," Anita sings to Maria in "A Boy Like That," as a way of warning Maria that she cannot trust someone outside of her racial group.
Adults vs. Kids
In addition to the tension between the Sharks and the Jets, there is a marked tension between the adolescent world of the gangs and the adult world. Lieutenant Schrank and Officer Krupke are trying to keep the peace and make sure the two gangs stop fighting, but they have no sensitivity to what the kids are going through and what kinds of prejudices the young people have inherited. In many ways, the tension between the two groups can be traced back to principles and bigotry that they learned from their elders, and the film seeks to expose the ways that adults are hypocritical when they urge the younger generation to be more tolerant. Indeed, Lieutenant Schrank is just as racist, if not more racist, than the young Jets.
Immigration
Immigration is also a major theme in the film, as the members of the Sharks gang are all Puerto Rican immigrants. At the start of the film, Maria has only lived in the mainland U.S. for a month, and she is eager to see what new freedoms and adventures living there will afford her. In the song "America," the Puerto Rican boys debate with the Puerto Rican girls about the pros and cons of being immigrants in America. While Anita and the girls insist that life is better in New York, Bernardo playfully bemoans the fact that, as Puerto Ricans, they are treated as second-class citizens.
Indeed, the anxiety about the Sharks, especially as it is expressed by adults like Lieutenant Schrank, has to do with a racial prejudice and a penchant for white supremacy. Schrank bemoans the fact that the Puerto Ricans are taking over the neighborhood, showing up in traditionally white establishments and depreciating the value of the neighborhood. This is a prime example of the kind of hatred with which the Puerto Rican Sharks contend on a daily basis.
Class
While the Jets and the Sharks draw sharp lines between their group identities and can only focus on their racial differences, they are united in the fact that they are all poor. The fact that they are members of the lower classes, coming from broken homes and neglectful families, only exacerbates the tension between them, and they are aware of the micro-differences in their respective classes. Bernardo at one point points out that a Puerto Rican makes far less money than an American for the same work. Meanwhile, the white Americans worry that the Puerto Ricans are taking over the neighborhood and will take opportunities away from them. It is these kinds of economic disparities that only heighten the gang warfare and the racial divisions.
Love
The film is, perhaps most importantly, a love story, a retelling of Shakespeare's Romeo and Juliet, in which two lovers from opposing families fall in love against the wishes of their respective clans. Maria and Tony's affair is pure and strong from the minute they lay eyes on one another at the dance, to such an extent that director Robert Wise blurs the rest of the action to show that the two lovers only have eyes for each other. From there, their romance blossoms rapidly, and they each profess their love that night, and are soon planning their wedding. Love is an essential thing in life, and Maria and Anita—in spite of their disagreements—eventually agree on the fact that "Your love is your life."
The love shared between Tony and Maria is in direct conflict with the violence of the gangs. At the end, Maria scolds the Jets and the Sharks, saying that it was their hate that killed Tony.
Violence
Violence hums underneath the action of the film, as Jets and Sharks vie for the neighborhood. From the opening moment, when we see the Jets wandering through various basketball courts and parking lots, snapping and whistling, we can tell that the gang culture in the neighborhood is an intimidating and threatening one. The boys, Sharks and Jets alike, have chips on their shoulder, and they can only reconcile their feelings by picking fights and antagonizing one another. Anita narrates that Bernardo builds up his feelings and can only calm down after a fight. Violence is something that haunts the gang members in the film, until it becomes a force for destruction and death.
Racial prejudice
A major theme of the film is racial prejudice. The Sharks face a great deal of racism and bigotry as non-white New Yorkers in the neighborhood. The Jets call them "Spics" and deem them untrustworthy time-and-again, and much of the tension surrounding Maria and Tony's affair has to do with Bernardo and Anita worrying that Maria cannot trust a white boy to take care of her best interests.
Racism is at its ugliest in the scene in which Anita visits Doc's store to deliver the message from Maria. The Jets surround her and spout racist vitriol at her, molesting, heckling, and objectifying her. Their dehumanization of Anita is an outcropping of their devaluation of her as a Puerto Rican and is unmistakably racially charged.
