Never one to hold his tongue, Seth Rogen explained why he tweeted some not-so-nice things about Justin Bieber after several encounters.

The “Neighbors” star showed up at “The Howard Stern Show” on Monday (May 5) and had no shortage of ammo to back up his derogatory claims about the troubled pop star.

Seth began, "I met him a few times. He's a good example of someone, who you meet, who you think you are going to hate and then you get to hate him ... you meet him and he lives up to every one of your expectations."

A few years back, Rogen shared, he ran into Bieber at a German talk show. "Someone came to the door and was like 'Justin Bieber wants to meet you.’” But when Seth came out of his dressing room, Justin acted like it was Rogen who had requested the meet-up. "What the f*ck, I don't want to meet you! Don't act all nonplussed to meet me. I didn't want to meet you."

Making matters worse, a few years later Seth bumped into the “One Less Lonely Girl” crooner again. "I saw him again at like an MTV awards show, he literally had a snake wrapped around his f--king wrist that he was wearing. I talked to him for like five minutes. ... No humility, no awareness, no sense that 'I appeal to young, young people, maybe a grown man who works in comedy doesn't f*cking give a sh*t about me. Maybe I should act in such a way that perhaps this isn't the greatest experience of his f*cking life.'"

Then, during a meeting Rogen tweeted that Justin was a “piece of sh*t.” "I remember thinking 'He's a piece of sh*t.’ Then I came out of the meeting and it had been retweeted like 200,000 times. I seemed to have tapped into something here."