David Dobrik thought he’d peaked after he married his best friend’s mom. His friend and podcast co-host, Jason Nash, had told him he would never find anyone to marry, and so, as payback, Dobrik became Nash’s stepfather. The quickie wedding, conducted at the campy Little White Chapel in Las Vegas, was filmed as an elaborate bit for Dobrik’s successful YouTube channel. (Six months later Dobrik instagrammed himself signing divorce papers.) Standing on a fire escape in New York City last May after returning from his honeymoon in Hawaii, the Los Angeles-based YouTuber waved to throngs of screaming fans who had assembled to...