Fun
Perhaps the most central theme of Animal House is fun, and the right of college students to pursue fun at the expense of their grades and all of the other academic obligations of college. The students in the film are hardly ever shown working hard or going to class. Rather, college is a time to experiment with partying and debauchery, to emphasize fun over hard work—which there is sure to be plenty of in the real world.
The defining characteristic of the Delta fraternity is their belief in fun and their enjoyment of life. They want to soak up as much fun as they can, and that includes partying their time at college away no matter the consequences. The characters' love of fun informs the tone of the film itself, which never takes itself too seriously and stays lighthearted throughout.
Growing up
Part of what makes up the conflict of the film is the fact that authorities at the university want the members of the Delta fraternity to grow up and take life more seriously, but they want to soak up the pleasures of youth. The antics of the Delta brothers is often juvenile and outrageous, and this, more than anything, is what peeves people like Doug Neidermeyer and Dean Wormer, authority figures who emphasize discipline and responsibility.
Additionally, the question of maturity comes up in Boon and Katy's relationship. She wants him to leave behind the juvenilia of Greek life and prove to her that he's ready for a mature relationship, but he seems to have no interest, eschewing intimacy and maturity for pranks and his fraternity time and time again.
Breaking rules
The kind of fun that the brothers of the Delta fraternity have is often contingent on breaking the rules. They drink to excess, keep a slovenly household, overstep boundaries with potential dates, and generally defy expectations and regulations. The identity of the Delta brothers is defined by their enjoyment of going against the prevailing culture and defying the pressures of conformity. Neither countercultural nor straitlaced, they are defined by their investment in chaos and breaking with convention.
Sex
A major theme of the film is sex, and many of the narrative threads follow characters as they seek out sex, face sexual humiliation, or in some cases, get laid. Larry is a virgin when he arrives at college and is on the lookout for a potential mate, but when he finds one, she passes out drunk and he must face a moral quandary of whether or not to rape her. Then later, when they do finally have sex, he learns that she is underaged. These scenarios are played for laughs; just when the virginal college fraternity brother thinks he is getting lucky, he is duped by circumstance.
Additionally, Otter is a notorious ladies' man on campus and scores with numerous women, including his nemesis' girlfriend Mandy, and the dean's wife, Mrs. Wormer. His sexual prowess is contrasted with that of Greg Marmalard, his nemesis, who is shown at various points getting masturbated by a beautiful woman, but failing to get erect.
Stupidity
One of the main tenets of the Delta fraternity is its investment in stupidity. Their wild antics are compelling not only because they seem like fun or because they break rules, but because they subvert stodgier situations and create an absurdist ethic, an ethic of stupidity, that ruptures the pomposity of their enemies. In his final rallying cry before the brothers disrupt the homecoming parade, Otter compels his brother by saying, "I think that this situation absolutely requires a really futile and stupid gesture be done on somebody's part!"
Structure and Discipline
The chaos and willful stupidity of the Delta brothers are in direct defiance of the prevailing order on campus, endorsed by Dean Wormer and conformists like Neidermeyer and Greg Marmalard. This order is militant and fascistic in its articulation, perhaps best encapsulated by the hotheaded Neidermeyer in a helmet barking orders at his ROTC cadets, or bragging to his fellow Omega brothers about his father's time in the military. Discipline and structure are the things that Delta brothers most resent and despise, and it is a looming villainous force in the film: the symbol of everything that the protagonists seek to elude.
Revenge
When the Delta boys finally get kicked off campus, expelled for good, they are dejected and alarmed that their freewheeling lifestyle is finally facing pushback. However, once Bluto and Otter muster some conviction to mobilize the brothers, they set to work plotting revenge. Their brand of revenge is not of the violent kind, but a revenge based on disruption and subversion. The final scene of the film is a cathartic outburst, as we watch the brothers of the Delta house clown their way into the homecoming parade and inject chaos into the proceedings.
