The hangover as a symbol
For a 19 year old to have a hangover and to cure it with marijuana is a symbolic portrait of drug and alcohol abuse, because it shows that he is using recreational drugs as medicine, medicating away the pain of one with the other. It suggest that he is also self-medicating with alcohol, making the hangover a symbol for mental frustration and clouded judgment. He needs to solve problems, but his coping mechanisms literally cloud his judgment.
Marriage as a complex symbol
To some people in the book, marriage is the answer to most of life's problems and questions, but when Ari sees people who believe that, like Joe, for instance, he takes a contrary stance. To him, marriage represents cowardice, because he doesn't feel that family is important except to the people who are too cowardly to do life alone, like he is. That means that the prospect of marriage is a symbol for his loneliness.
Paranoia and drugs, a motif
One of the side effects of illegal drug usage is extreme paranoia, especially if the drug amplifies one's emotions, like marijuana, or if the drug amplifies one's energy level (like adrenaline). When the protagonist buys speed, that is part of a larger motif that points the reader to the character's chronic paranoia. He is an outsider who lives beyond the rules of convention, so the motif points the reader to chaos, because he has rejected the social rules of law and convention.
Homosexuality and health
Ari experiences attraction to same-sex partners, like the Greek man in the alleyway, like the gay man in the bar, and like George, whom Ari likes. These are a motif that show that Ari's experience of homosexuality is not exactly healthy, because he is often very frustrated by his sexual attractions. He is not healthy because he chooses to hide his sexual orientation, leading to even more paranoia. He is attracted to men, but he isn't necessarily comfortable accepting himself for that.
Parents as divine symbols
Ari realizes that his parents will be angry in the novel's ending. The mother and father are at church, which helps to form this symbolic idea, that parents are like living metaphors for divine judgment, because of the role that parents play in the developing psychology of a child. The fact that they are religious helps the reader to know why Ari doesn't accept his own homosexuality, because he thinks that his parents' opinions are like God's opinion or something. In the end, he meditates on accident, which shows that he needs to think through his relationship to their authority.
