Clark, the smart, judgmental teen at the heart of “Quitters,” has a reason for being that way.

His mother (Mira Sorvino) is a medicated mess, weepy, unstable and needy. She insists on driving him to a friend’s house after she’s canceled a family getaway at the last minute. She’s in her bathrobe. She’s fawning over him, utterly distracted despite his “Watch the road, Mom,” pleas.

When she plows into a stop sign, it’s her turn to plead. “Let’s keep this between us.”

Whipping out his cellphone tells us he’s not having it. “I’d like to report an accident.”

Clark quits on her, once and for all, it seems.

“Quitters” plays as a downward spiral that plainly has been underway for a while.  Mom is off to rehab, where they’ll try to get her medications right. Because she needs them. Clark, given the loathsome certitude of the self-righteous by Ben Konigsberg (“Anesthesia”), hears his dad (Greg Germann) beg him to “keep this quiet.”

But Clark is above that kind of reasoning. The girl (Kara Hayward) that he most wants to impress gets an earful. And when she dismisses his romantic overtures — with extreme prejudice — she gets Clark’s unfiltered take, too. He starts a whispering campaign about her “depression,” and informs her, by condescending email, that he’s A) “concerned” and B) can no longer “be your friend.”

He’s the sort of teen who debates his hip, young English teacher (Kieran Culkin, quite good) about his grade on an essay, and bullies the guy to get his way.

The kid finds Dad’s “chipper” (small marijuana pipe) in the glove compartment, and wrecks it. He sees a receipt for a massage parlor, and in front of his father, calls the place to catch the old man in an indiscreet lie.

First-time feature writer-director Noah Pritzker has created a near-classic anti-hero in Clark, a kid who wins our sympathy, then our fury and finally, something resembling our pity in this 93 minute film.

We wince at Clark’s infuriating mix of tactlessness and cluelessness. “Awkward” doesn’t begin to cover it. And we grimace at his every misstep, because we see them coming long before he does.

He ogles a hot mom (Saffron Burrows), only to ingratiate himself with her daughter, a classmate (Morgan Turner) he barely knows. He joins them for dinner and angles his way into their lives and into classmate Natalia’s bed, even though she seems to see through him.

He punishes his father, and watching Mom’s narcissistic approach to rehab, we wonder if Dad actually deserves any of this. He sees his son as “a mean spirited little s—.”

We will, too.

Konigsberg is deftly infuriating as Clark, Hayward and Turner make their vulnerable characters more insecure girls next door than beguiling teen sirens, Burrows suggests a deep soul with a dark side and Germann just looks….guilty. Of something.

Set in San Francisco’s tony Presidio, idle affluence permeates Pritzker’s picture. The title “Quitters” signifies relationships that one and all are quick to abandon — the parents cheat or have cheated, the kids abandon this or that significant person in their lives on an impulse. The parents have quit on their kids, too. Sorvino’s May is merely the last to do that.

There’s an abruptness to the conclusion that makes “Quitters” feel incomplete. But Pritzker has conjured up a world and peopled it with believable co-dependents, no mean feat in a 93 minute film.

The adult “Quitters”, caught up in their own melodramas, never ponder how their kids turned out this way, which is funny. Narcissists, by nature, are never that far from a mirror.

MPAA Rating: Unrated, with pot use, teen sex, exploitative sexual situations, profanity

Cast: Ben Konigsberg, Kara Hayward, Morgan Turner, Mira Sorvino, Kieran Culkin, Greg Germann, Saffron Burrows

Credits: Written and directed by Noah Pritzker. A Monument/eOne release.

Running time: 1:33