Recently, foul-mouthed philosopher king Louis C.K. made an appearance on Conan O'Brien's show, where he discussed why he doesn't want his daughters to have cell phones. His reason boils down to this: if you're always entertained, and connected with other people, and distracted, and so on, then you never come face-to-face with what C.K. calls the "Forever Empty"—the innate human capacity for crushing loneliness and existential fear and dread and, above all, hopeless solipsism, the feeling like we're so profoundly disconnected from everything else there is that there's no hope of even trying to reach out to someone else for any purpose beyond self-gratification.
(One of Jon's women says, "When you have sex with me, it's like I'm not even there. Like I don't even exist.")
C.K.'s point is that we, as humans, engage in myriad self-destructive behaviors because they allow us to escape, with dental-dermal closeness, from experiencing real loneliness and isolation.
C.S. Lewis made a similar point, though it's frequently and incorrectly twisted to be saccharine. When Lewis claims, "If I find in myself desires which nothing in this world can satisfy, the only logical explanation is that I was made for another world"—yes, one response to that is to be happy at the idea of being whole and satisfied.
But what's so frequently omitted from the discussion is that we're still in this world, even if we're not of it. That our entire life will be lived with this feeling of a thing inside us that feels like loneliness, or emptiness, or fear, or hurt. That feeling drives all of Don Jon, start to messy finish, every character pursuing in some way what he or she sees to be fullness of life. And all of them come up short.
Don Jon can link its metaphysical and emotional roots, as such, to some of the best work of P.T. Anderson and David Foster Wallace, particularly in Anderson's 2012 film The Master and Wallace's 1999 short story collection Brief Interviews With Hideous Men. (The film has more links to Anderson, too: Julianne Moore's casting seems to reference to her similarly Jocastian character in Boogie Nights, and the film cribs stylistically from the more formally inventive moments of Magnolia.)
All these works have a complex core obsession, as phrased earlier: why do we feel so alone? What's keeping us from—as Jon repeats over and over—"losing ourselves" in another human? And how do we stop feeling that way? And what do we do to change?