Even if they're not dbags in the first place, being a celebrity gives you more opportunities to act out than having a normal job, meaning that even a nice guy might growl at someone at the adjacent urinal if they asked for an autograph. Just like the majority of celebrities, the majority of fans must be terrible, too. Even my micro-inkling of Internet fake-fame has shown me countless "fans" who thought they could endear themselves by being combative assholes. I had one guy criticize my family and not understand why he got de-friended on Facebook. He's probably reading this now and telling his foreman at the jackass factory, "Hey, that a*****e he's talking about is me!!! I'm that a*****e."
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Still not clear if the "jackass factory" employs jackasses or actually manufactures them as well.