Harry Styles' 21st birthday bash was certainly star-studded, as even former model flings Kendall Jenner and Cara Delevingne were there -- but how come his other ex-girlfriend, Taylor Swift, was not invited?

Swift was noticeably missing at Lulu's Café in Beverly Hills over the weekend. According to HollywoodLife.com, she was intentionally kept off the guest list to avoid any drama.

"She was never invited because he wanted to have fun, and inviting drama and all the nonsense she would bring is never fun. Why bring oil in a party full of water?" a source revealed.

The One Direction member apparently did not want to be distracted by the "Blank Space" singer's diva-like attitude.

"She would want to be the center of attention, and it was his birthday," the source continued. "He wants to be the master of ceremonies so to speak, he doesn't want to babysit her and have to watch who she is talking to."

"He didn't want to make his birthday something more than what it was, by adding Taylor it would have added so many elements to the night an he wanted to have fun," the source added. "Taylor being there would have meant he would have had no fun, simple as that."

As he planned, Styles had a blast at his lavish party, especially with his current rumored girlfriend Nadine Leopold.

"Everyone's teasing him, saying he's in love, and he's not denying it. It's the real deal," an insider revealed to The Sun (via Mirror). "He's never felt this way about anyone before."

"Harry has spent virtually all his free time in Los Angeles with Nadine. They're infatuated with each other and can't stand being apart," the source added. "There's nobody else he'd rather be with on his birthday."