The first scene of Don Jon (written and directed by Joseph Gordon-Levitt, who also stars) features "The Don" Jon and two of his friends standing in a club, evaluating the appearance of every woman there. It's the pronouns that are important here: "It's a dime!" says one of Jon's friends (meaning a perfect ten out of ten). "Naw," Jon responds, "Maybe it's an eight. Maybe."
And so every viewer in the audience gears up for an object lesson about the Dangers of Objectifying Women, or the Soul-Withering Effects of Pornography, or something—but Don Jon is about something bigger than an addiction to pornography. There's never a moment where his porn addiction is treated like a "sin" in anything other than a nominal sense, as when Jon describes his actions to a faceless priest. No, for Levitt, porn is just a case study for what the film really cares about: why do we feel so alone?
Jon sleeps with different women every night, but still slips out of the arms of the woman beside him to go and watch porn. "I think I like porn better than [sex]," he says (though employing a more vulgar term). "I just lose myself in it, in a way I don't when I'm [having sex]." But when Jon decides it's time to get a girlfriend, Barbara (Scarlett Johansson), he realizes that she has her escapist fantasies, too. She sits in the theater and watches (staring at the camera wide-eyed and rapturous) as the film-within-a-film follows your traditional rom-com structure: meet, fall in love, break up, get back together, get married, ride off into the sunset. The cinematography emphasizes what Jon later makes explicit, at least in thought: "You escape into your movies just as much as I escape into porn."
At home, Jon's dad (Tony Danza) can't go five seconds without football on the TV; his mom (Glenne Headley) tells stories from when she and Danza were young, trying to reconstruct a fantasy that's pretty heavily implied never existed. In what becomes a five-time running gag, Jon's sister (Brie Larson) is never without her cell phone, even in the middle of Mass, or dinner, and never (with one exception) interacts with the family in any way.
And then there's Esther (Julianne Moore), an adult learner in Jon's college class, whose secret is that she cries alone, a lot (maybe as much as every half hour or so, by her own estimates). Any time Esther is outside of class, she is either smoking weed, or having sex, or crying; there is nothing else, anymore, for her.
Which means that maybe the first criticism you can level against Don Jon—that it's just slightly too on-the-nose, all the time, with lines like "I could quit watching porn if I wanted to," or entire conversational digressions about what void it fills in his life—is pretty much invalid, in a way it wouldn't be if the movie was just about the Dangers of Porn Addiction. Levitt delves so deeply into Jon's central fixation because it illuminates everything at the fringes of the movie. It's through the lens of Jon's absolute all-consuming desire to "lose himself" in pornography that his father's television habits make more sense, or his sister's ceaseless texting, or Barbara's obsession with the idea that "When a man loves you, he won't mind doing anything for you—that's what love is."