The Master, Brief Interviews, and Don Jon all pose the same answer to the question of "what do we do to solve the problem of loneliness": sex. "Making love." Feeling whole with someone else requires physically being whole with someone else. It's the closing shot of both The Master and Don Jon, and Brief Interviews ends on a story that frames a similar encounter, of being made whole through sexual contact with someone who's able to connect with you. All three works draw a fairly sharp distinction between just sex—between the kind of sex conjured up by the sorts of slurs Jon and his friends use at the beginning of the movie—and "making love," a phrase Jon employs in an adorably un-ironic way at the end of the movie.
What I imagine some people will miss if they attack Don Jon for its answer to this question, and for its constant stream of fairly graphic sexual content, is that if we say that the answer the movie provides is "short-sighted" or incomplete or incorrect or something, we are obligated to present an alternative.
In the mammarial frenzy of movies like Don Jon, it's easy (in the heat of righteous indignation) to forget that for hundreds of thousands of people walking around every day in just your city alone (assuming yours is a small city), sex really is the best answer they have. So striking at the content of the film as "immoral" or "detestable" or whatever may not be technically incorrect, but it also profoundly misses the point, and is at best unhelpful.
Don Jon chronicles the journey of a man who moves from implicitly nihilistic hedonism to something more communal, more shared, and more loving. Just because his growth is incomplete doesn't mean that the whole work is false or forfeit. After all, we live in a culture that teaches us, 24/7, day in and day out, that men are there to have sex and women are there to have sex with, that sex is something you do to have fun, and that connection and love and emotional bonding are all cute but outdated concepts, that really everything is empty and hollow and so you might as well enjoy the hollow empty party because the lights go out at midnight, and there's nothing after that.
Don Jon tries to suggest an alternative. And even if it's a deliberately incomplete answer, even if it's surrounded by deep trenches of sexual content, even if we don't entirely agree—I want to champion it and call it good, if just not enough. Some art revels in its own absent heart, in the hollowness of everything presented, in pointing out that, as an artist, they're just a jester in a broken and irreparable system. But then some art sees that something is wrong, that we're having a cultural heart attack, and tries to diagnose us, to help re-orient our hearts and minds around something better.
Don Jon is staunchly the latter, and deserves recommendation on those merits alone: for being true to life, even when life is ugly and sexual and gross, but also for not stopping there. It is not content to just entertain itself with or take advantage of the present brokenness of the world, but tries to point upwards or outwards or something, to somewhere else, even with an undefined or crude notion of where that place is.