On Monday, former New York Congressman Anthony Weiner was sentenced to 21 months in prison for having lewd exchanges with a 15-year-old girl on Skype and Snapchat. The sentencing was just the latest act in Mr. Weiner’s nightmarish downfall fueled by his compulsive sexting habit.

Mr. Weiner cried as he pleaded guilty to one count of transferring obscene material to a minor, a crime that carried up to 10 years in prison. “I acted not only unlawfully but immorally, and if I had done the right thing, I would not be standing before you today,” he told Judge Denise L. Cote in Manhattan.

“I was a very sick man for a very long time,” he said. Indeed, his behavior fits a commonly understood definition of addiction, which is persistent engagement in a behavior despite negative consequences. And those consequences were epic — loss of his congressional office in 2011, the implosion of his mayoral bid in 2013, public humiliation, the dissolution of his marriage and accusations of jeopardizing Hillary Clinton’s presidential bid.