"Of course it stings a bit that Kourt is hooking up with Bieber. But Scott also knows his place and that he doesn't have the right to have a fit about it," a source tells PEOPLE. "He is convinced Kourt is doing it to hurt him. And she succeeded."
The singer, 21, and the Keeping Up with the Kardashians star, 36, were spotted being "super touchy-feely," at the Nice Guy in West Hollywood on Friday, sparking rumors of a romance.
Other aspects of Disick's life are on the upswing after he entered rehab in October, the source says.
"Otherwise Scott is doing well and continues to focus on his sobriety. It's no problem for him to be out with friends and not drink," the source says of the father of three. "He feels great not drinking. He loves spending time with his kids."
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A source previously told PEOPLE that while Kardashian and Bieber have been hooking up, it's nothing serious.
"It's been a deep flirtation for months. He's obviously close to the family, and Kris [Jenner] is obsessed with him," a source previously told PEOPLE. "It's just one of those things where it makes Kourtney feel good about herself. It's not serious at all. This young guy is into her, he thinks she's hot and she's older. She just wants to have fun."
Disick, who split from Kardashian in June, was spotted partying Tuesday at a listening party for Chris Brown's new album "Royalty" at Hyde in Los Angeles. Kardashian's younger sister Kendall Jenner happened to be there, but she gave him the cold shoulder, a source told PEOPLE.