Seth Rogen is surprisingly taking the blame for his more than year-long beef with Justin Bieber.

The 33-year-old funnyman appeared on "The Ellen DeGeneres Show" airing Wednesday -- coincidentally during "Bieber Week" to celebrate the singer's new album, "Purpose" -- when he said that he patched things up with the 21-year-old pop star backstage. Last January, Rogen tweeted that Bieber was a "piece of s--t," and had even less flattering things to say when he called him a "d--k" on Howard Stern's SiriusXM show in May of last year.

"It's not really a beef, I just called him some mean things on social media for no real reason," Rogen laughed. "And I'd only met him very briefly. I can't tell you why I did it, I was inclined to do it at that moment, and then I did it."

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"And then it really hurt his feelings -- as it should have -- and a lot of people got very mad at me," he continued. "I finally saw him today in the hallway of your show and I guess we made up, which is fantastic. It's all my fault, it's all because I decided to be mean one day for no reason whatsoever."

Rogen even complemented Bieber's recent nude pics. "In light of recent photos, I want him on my side," the "Neighbors 2" star quipped.

Rogen's new attitude on Bieber is a big change from last May, when he slammed Bieber during his candid chat with Stern. "He's a good example of someone you meet, who you think you're going to hate him, and then you get to hate him as much as you thought," Rogen said. "You meet him, and he meets up to every one of your [bad] expectations of how you hope he will be."

" ... I was like, 'Aye, he's a bit of a mother----er. He's young, the kid's a d--k,'" he added about their meeting.

Bieber, however, seemed to take the diss in stride.

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"Seth Rogan sorry I didn't bow down when I asked 2 meet u was probably a bit shy and didn't want to be over the top but still. love ur movies," Bieber later responded. He also begged the comedian to roast him during his Comedy Central Roast in March, but got no response back.

Though one heartthrob Rogen is obviously a big fan of is his "Neighbors 2" costar, 28-year-old Zac Efron. The "Night Before" actor had nothing but nice things to say about the handsome star, except that he absolutely hated being shirtless next to him.

"That kid is a winner," he smiled. "He's even more handsome this time around. I don't know how that happened. It's something he eats. It's crazy. He gets better and better looking. He's aging like a fine wine."

"And I have to be shirtless next to him, and it's horrible -- just horrible," he joked. "It's your worst nightmare."